F99 Mental disorder, not otherwise specified

The ICD-10-CM code for Mental disorder, not otherwise specified is F99 (FY2026). It is a billable, claim-ready diagnosis code.

Classification

Chapter

5: Mental, Behavioral and Neurodevelopmental disorders (F01-F99)

Section

F99: Unspecified mental disorder (F99)

Category F99

1 code (1 billable)

FY2026 Status

Stable since FY2024

Also Known As

ICD-10-CM Alphabetic Index entries that lead to F99:

• Psychiatric disorder or problem
• Situation, psychiatric
• Disease, diseased › mental
• Disorder (of) › mental (or behavioral) (nonpsychotic)
• Disorder (of) › psychological
• Disease, diseased › psychiatric
• Aberration › mental
• Disturbance (s) › mental

Inclusion Terms

• Mental illness NOS

U.S. Hospital Utilization

• An estimated 6,370 U.S. inpatient stays in 2023 included F99 among the documented diagnoses.
• 175 stays listed it as the principal diagnosis.

Source: National Inpatient Sample (NIS), Healthcare Cost and Utilization Project (HCUP), Agency for Healthcare Research and Quality, 2016–2023. National survey-weighted estimates.

Official Coding Guidelines

Chapter 5 TOC entry

5. Chapter 5: Mental, Behavioral and Neurodevelopmental disorders (F01 - F99) ................... 43

Pain disorders related to psychological factors (TOC entry)

a. Pain disorders related to psychological factors ..................................................................... 43

Source: CMS — ICD-10-CM Official Guidelines for Coding and Reporting, FY2026

References

• CMS — ICD-10-CM Official Code Set and Guidelines, FY2026
• CDC/NCHS — ICD-10-CM Files and Browser Tool
• AHRQ — HCUP National Inpatient Sample, Frequencies by Diagnosis Code (2016–2023)