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ICD-10-CM 2027 diagnosis code

I66.9Occlusion and stenosis of unspecified cerebral artery

✓ Billable / specific

I66.9 is a valid, billable ICD-10-CM code for occlusion and stenosis of unspecified cerebral artery. It can be reported on claims for dates of service from October 1, 2026 – September 30, 2027.

Looking up by condition? See ICD-10 code for Intracranial arterial thrombosis.

Code unchanged since ICD-10-CM took effect on October 1, 2015. Source: official FY2027 ICD-10-CM release from CDC/NCHS and CMS. Page updated September 29, 2026. About our data

About I66.9

I66.9 is the ICD-10-CM diagnosis code for occlusion and stenosis of unspecified cerebral artery. It belongs to category I66 (occlusion and stenosis of cerebral arteries, not resulting in cerebral infarction), block I60-I69 (cerebrovascular diseases) and chapter 9 (diseases of the circulatory system). It is billable, so it can be reported on its own for dates of service from October 1, 2026 through September 30, 2027.

Within I66, choose I66.9 (unspecified cerebral artery) only when documentation doesn't support a more specific option: middle cerebral artery (I66.0), anterior cerebral artery (I66.1), posterior cerebral artery (I66.2), cerebellar arteries (I66.3) and other cerebral arteries (I66.8).

It is not a CC or MCC, so as a secondary diagnosis it doesn't change the MS-DRG severity level.

In MS-DRG v44.0, it is part of the grouping logic for DRG 061 (Ischemic Stroke, Precerebral Occlusion or Transient Ischemia with Thrombolytic Agent with MCC, relative weight 2.7211), DRG 062 (Ischemic Stroke, Precerebral Occlusion or Transient Ischemia with Thrombolytic Agent with CC, relative weight 1.7781), DRG 063 (Ischemic Stroke, Precerebral Occlusion or Transient Ischemia with Thrombolytic Agent without CC/MCC, relative weight 1.455), DRG 067 (Precerebral Occlusion without Infarction with MCC, relative weight 1.4274), DRG 068 (Precerebral Occlusion without Infarction without MCC, relative weight 0.9013), DRG 791 (Prematurity with Major Problems, relative weight 4.1075) and 1 more, in MDC 01 (Diseases and Disorders of the Nervous System) and MDC 15 (Newborns and Other Neonates with Conditions Originating in the Perinatal Period), as a principal or secondary diagnosis. The DRG a claim lands in depends on the principal diagnosis, procedures and any CC/MCC secondary diagnoses.

It has been part of ICD-10-CM since the code set took effect on October 1, 2015 and hasn't changed since.

Before ICD-10, this condition was coded in ICD-9-CM as 434.10 (cerebral embolism without mention of cerebral infarction) and 434.90 (cerebral artery occlusion, unspecified without mention of cerebral infarction).

Notes that apply from higher levels

Instructions written at a parent level also apply to I66.9.

› From I66 Occlusion and stenosis of cerebral arteries, not resulting in cerebral infarction
Includes
  • embolism of cerebral artery
  • narrowing of cerebral artery
  • obstruction (complete) (partial) of cerebral artery
  • thrombosis of cerebral artery
Excludes1
  • Occlusion and stenosis of cerebral artery causing cerebral infarction (I63.3-I63.5)

Broader instructions also apply from I60-I69 Cerebrovascular diseases and Chapter 9: Diseases of the circulatory system.

Alphabetic index entries

4 entries

Terms in the official ICD-10-CM index that lead to I66.9.

MS-DRG v44.0 grouping

All DRGs →

Inpatient MS-DRGs this diagnosis can group to, as the principal or a secondary diagnosis, depending on the rest of the claim.

MDC 15 · Newborns and Other Neonates with Conditions Originating in the Perinatal Period

MDC 15 · Newborns and Other Neonates with Conditions Originating in the Perinatal Period

ICD-9-CM equivalent

Converter →
  • 434.10Cerebral embolism without mention of cerebral infarctionapproximate
  • 434.90Cerebral artery occlusion, unspecified without mention of cerebral infarctionapproximate

From the CMS 2018 General Equivalence Mappings (GEMs), the final GEMs release.

Code history

  1. 2016
  2. 2017
  3. 2018
  4. 2019
  5. 2020
  6. 2021
  7. 2022
  8. 2023
  9. 2024
  10. 2025
  11. 2026
  12. 2027
  • FY2016 (effective 10/1/2015): Added (first year of ICD-10-CM)
  • No changes since FY2016.

Common questions about I66.9

Is I66.9 billable?

Yes. I66.9 is a billable/specific ICD-10-CM code valid for FY2027 (October 1, 2026 – September 30, 2027).

Can I66.9 be used as a principal diagnosis?

Yes. The Medicare Code Editor doesn't restrict I66.9 as a principal diagnosis.

Is I66.9 a CC or MCC?

No. I66.9 is neither a CC nor an MCC under MS-DRG v44.0.

What DRG does I66.9 group to?

I66.9 is used in the MS-DRG v44.0 logic for MS-DRG 061 (Ischemic Stroke, Precerebral Occlusion or Transient Ischemia with Thrombolytic Agent with MCC), MS-DRG 062 (Ischemic Stroke, Precerebral Occlusion or Transient Ischemia with Thrombolytic Agent with CC), MS-DRG 063 (Ischemic Stroke, Precerebral Occlusion or Transient Ischemia with Thrombolytic Agent without CC/MCC), MS-DRG 067 (Precerebral Occlusion without Infarction with MCC), MS-DRG 068 (Precerebral Occlusion without Infarction without MCC), MS-DRG 791 (Prematurity with Major Problems) and 1 more. Which one applies depends on whether it is the principal diagnosis and on the rest of the claim.

What is the ICD-9 code for I66.9?

The CMS General Equivalence Mappings map I66.9 to ICD-9-CM 434.10 and 434.90.