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ICD-10-CM 2027 diagnosis code

F99Mental disorder, not otherwise specified

✓ Billable / specific

F99 is a valid, billable ICD-10-CM code for mental disorder, not otherwise specified. 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 Situation, ICD-10 code for Mental disorder, ICD-10 code for Psychological disorder, ICD-10 code for Psychiatric disorder or problem.

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 F99

F99 is the ICD-10-CM diagnosis code for mental disorder, not otherwise specified. It belongs to block F99 (unspecified mental disorder) and chapter 5 (mental, Behavioral and Neurodevelopmental disorders). It is billable, so it can be reported on its own for dates of service from October 1, 2026 through September 30, 2027.

Don't report F99 together with the conditions in its Excludes1 note: unspecified mental disorder due to known physiological condition (F09).

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 880 (Acute Adjustment Reaction and Psychosocial Dysfunction, relative weight 0.9306), in MDC 19 (Mental Diseases and Disorders), 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 300.9 (unspecified nonpsychotic mental disorder).

Coding notes

Applicable toConditions and synonyms classified to this code
  • Mental illness NOS
Excludes1Not coded here — never report together with this code
  • unspecified mental disorder due to known physiological condition (F09)

Notes that apply from higher levels

Instructions written at a parent level also apply to F99.

Broader instructions also apply from Chapter 5: Mental, Behavioral and Neurodevelopmental disorders.

Alphabetic index entries

10 entries

Terms in the official ICD-10-CM index that lead to F99.

Codes whose notes reference F99

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.

ICD-9-CM equivalent

Converter →
  • 300.9Unspecified nonpsychotic mental disorderapproximate

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 F99

Is F99 billable?

Yes. F99 is a billable/specific ICD-10-CM code valid for FY2027 (October 1, 2026 – September 30, 2027).

Can F99 be used as a principal diagnosis?

Yes. The Medicare Code Editor doesn't restrict F99 as a principal diagnosis.

Is F99 a CC or MCC?

No. F99 is neither a CC nor an MCC under MS-DRG v44.0.

What DRG does F99 group to?

F99 is used in the MS-DRG v44.0 logic for MS-DRG 880 (Acute Adjustment Reaction and Psychosocial Dysfunction). 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 F99?

The CMS General Equivalence Mappings map F99 to ICD-9-CM 300.9.