2026 ICD-10-CM Diagnosis Code G04.00Acute disseminated encephalitis and encephalomyelitis, unspecified

ICD-10-CM Codes›G00–G99›G00-G09›G04

ICD-10-CM G04.00
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

G04.00 is a billable ICD-10-CM diagnosis code for acute disseminated encephalitis and encephalomyelitis, unspecified. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026). Coders also document this condition as acute disseminated encephalomyelitis. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Encephalitis.

Code Identity

ICD-10-CM Code
G04.00
Billable Status
Yes — Valid for Submission
Code Describes
Acute disseminated encephalitis and encephalomyelitis, unspecified
Short Description
Acute disseminated encephalitis and encephalomyelitis, unsp
Parent Code
G04.0
Acute disseminated encephalitis and encephalomyelitis (ADEM)

Code Classification

ChapterG00–G99Diseases of the nervous system
SectionG00-G09Inflammatory diseases of the central nervous system
CategoryG04Encephalitis, myelitis and encephalomyelitis
This CodeG04.00Acute disseminated encephalitis and encephalomyelitis, unspecified

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Acute disseminated encephalomyelitis
  • Demyelination of spinal cord
  • Multiphasic acute disseminated encephalomyelitis
  • Recurrent acute disseminated encephalomyelitis

Index to Diseases and InjuriesGuidance

Alphabetical index entries that point to this code.

External Cause of Injuries IndexGuidance

References for this code in the External Cause of Injuries Index.

    • Encephalitis(chronic) (hemorrhagic) (idiopathic) (nonepidemic) (spurious) (subacute)
      • acute
        • disseminated
    • Encephalitis(chronic) (hemorrhagic) (idiopathic) (nonepidemic) (spurious) (subacute)
      • disseminated, acute
    • Encephalomyelitis
      • acute disseminated
    • Meningoencephalomyelitis
      • acute NEC (viral)
        • disseminated

Clinical ClassificationClinical

AHRQ’s CCSR groups this code into broader clinical categories.

CCSR NVS002
Encephalitis
Default principal diagnosis: inpatient Yes · outpatient Yes

Patient EducationClinical

Encephalitis

Encephalitis is inflammation (swelling) of the brain. It can happen when an infection or medical condition that affects the brain activates your immune system. The inflammation can cause a wide range of symptoms. In extreme cases, it can lead to brain damage, stroke, or even death.

The full article covers:

  • What is encephalitis?
  • What causes encephalitis?
  • Who is more likely to get encephalitis?
  • What are the symptoms of encephalitis?
  • How is encephalitis diagnosed?
  • What are the treatments for encephalitis?
  • Can encephalitis be prevented?

Read the full article at MedlinePlus

Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.

Convert G04.00 to ICD-9-CMHistory

The closest ICD-9-CM equivalents under the General Equivalence Mappings.

ICD-9-CM
323.61 Inf ac dis encephalomyel
Approximate The match is approximate rather than exact.

Code HistoryHistory

FY 2016AddedAdded to the ICD-10-CM code setEffective October 1, 2015, the first year of ICD-10-CM.
FY 2017–2025No changes
FY 2026CurrentCurrent code set, no changesEffective October 1, 2025 through September 30, 2026.

Questions About G04.00Overview

Is G04.00 (Acute disseminated encephalitis and encephalomyelitis (ADEM)) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report acute disseminated encephalitis and encephalomyelitis, unspecified on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What is the ICD-9 equivalent of G04.00?

Under the General Equivalence Mappings, acute disseminated encephalitis and encephalomyelitis, unspecified converts to ICD-9-CM 323.61 (inf ac dis encephalomyel). The mapping is approximate, so confirm the match fits the documentation.

Footnotes

[1] Not chronic - A diagnosis code that does not fit the criteria for chronic condition (duration, ongoing medical treatment, and limitations) is considered not chronic. Some codes designated as not chronic are acute conditions. Other diagnosis codes that indicate a possible chronic condition, but for which the duration of the illness is not specified in the code description (i.e., we do not know the condition has lasted 12 months or longer) also are considered not chronic.