2026 ICD-10-CM Diagnosis Code I40.9Acute myocarditis, unspecified

ICD-10-CM Codes›I00–I99›I30-I5A›I40

ICD-10-CM I40.9
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

I40.9 is a billable ICD-10-CM diagnosis code for acute myocarditis, unspecified. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 314 through 316, 974 through 976. As a secondary diagnosis, it counts as a major complication or comorbidity (MCC) and places an inpatient stay in the highest severity level of its MS-DRG family. It does not count, however, when the principal diagnosis is one of 22 closely related codes. Coders also document this condition as acute myocarditis. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Myocarditis and cardiomyopathy.

Code Identity

ICD-10-CM Code
I40.9
Billable Status
Yes — Valid for Submission
Code Describes
Acute myocarditis, unspecified
Short Description
Acute myocarditis, unspecified
Same as the full description in the CMS dataset.
Parent Code
I40
Acute myocarditis

Code Classification

ChapterI00–I99Diseases of the circulatory system
SectionI30-I5AOther forms of heart disease
CategoryI40Acute myocarditis
This CodeI40.9Acute myocarditis, unspecified

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Acute myocarditis
  • Interstitial myocarditis
  • Subacute interstitial myocarditis

Index to Diseases and InjuriesGuidance

Alphabetical index entries that point to this code.

Clinical ClassificationClinical

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

CCSR CIR005
Myocarditis and cardiomyopathy
Default principal diagnosis: inpatient Yes · outpatient Yes

Clinical InformationClinical

  • Acute Myocarditis

    the sudden onset of inflammation of heart muscle with myocellular necrosis; this is generally secondary to an infectious cause, and patients often have a recent history of a flu-like illness.

  • Interstitial Myocarditis

    inflammation of the heart characterized by infiltration of the interstitial tissues by inflammatory cells, histiocytes, and the formation of granulomas. giant cells are often present.

Patient EducationClinical

Cardiomyopathy

Cardiomyopathy is the name for diseases of the heart muscle. These diseases enlarge your heart muscle or make it thicker and more rigid than normal. In rare cases, scar tissue replaces the muscle tissue.

Read the full article at MedlinePlus

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

Convert I40.9 to ICD-9-CMHistory

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

ICD-9-CM
422.90 Acute myocarditis NOS
Exact Match The mapping is direct, with no qualifiers.

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 I40.9Overview

What is the ICD-10 code for acute myocarditis, unspecified?

The ICD-10-CM code for acute myocarditis, unspecified is I40.9 (sometimes written as I409). It is billable on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

Is I40.9 (Acute myocarditis) a billable code?

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

What MS-DRG does I40.9 group to?

When acute myocarditis, unspecified is the principal diagnosis on an inpatient stay, it groups to MS-DRG 314, 315, 316, 974, 975, 976, with relative weights from 0.6821 to 2.8860 depending on complications. Higher weights mean higher Medicare reimbursement.

Is I40.9 a CC or MCC?

CMS lists I40.9 as an MCC (major complication or comorbidity) for FY 2026. Reported as a secondary diagnosis, it places the inpatient stay in the highest-weighted DRG of its severity family. It does not count when the principal diagnosis is one of the 22 closely related codes in its exclusion list.

What is the ICD-9 equivalent of I40.9?

Under the General Equivalence Mappings, acute myocarditis, unspecified converts to ICD-9-CM 422.90 (acute myocarditis NOS). The mapping is a direct match.