R99 Ill-defined and unknown cause of mortality

The ICD-10-CM code for Ill-defined and unknown cause of mortality is R99 (FY2026). It is a billable, claim-ready diagnosis code.

Classification

Chapter

18: Symptoms, signs and abnormal clinical and laboratory findings, not elsewhere classified (R00-R99)

Section

R99: Ill-defined and unknown cause of mortality (R99)

Category R99

1 code (1 billable)

FY2026 Status

Stable since FY2024

Also Known As

ICD-10-CM Alphabetic Index entries that lead to R99:

• Death (cause unknown) (of) (unexplained) (unspecified cause)
• Sudden infant death syndrome (SIDS)
• Cot death
• Crib death
• Syndrome › crib death

Inclusion Terms

• Death (unexplained) NOS
• Unspecified cause of mortality

U.S. Hospital Utilization

• An estimated 470 U.S. inpatient stays in 2023 included R99 among the documented diagnoses.
• 215 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 18 scope; symptoms pointing to specific dx classified elsewhere

18. Chapter 18: Symptoms, signs, and abnormal clinical and laboratory findings, not elsewhere classified (R00-R99) Chapter 18 includes symptoms, signs, abnormal results of clinical or other investigative procedures, and ill-defined conditions regarding which no diagnosis classifiable elsewhere is recorded. Signs and symptoms that point to a specific diagnosis have been assigned to a category in other chapters of the classification.

R99 only when patient brought in already dead and pronounced on arrival

h. Death NOS Code R99, Ill-defined and unknown cause of mortality, is only for use in the very limited circumstance when a patient who has already died is brought into an emergency department or other healthcare facility and is pronounced dead upon arrival. It does not represent the discharge disposition of death.

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)