Although global knowledge on paediatric cardiomyopathies has advanced, prospective cohort studies from Brazil, particularly those integrating clinical and genetic data, remain limited.

To describe the clinical and genetic characteristics of paediatric cardiomyopathy patients and identify mortality predictors in a metropolitan region of Brazil.

Prospective observational study of paediatric patients with cardiomyopathies. Clinical data, genetic findings, and survival were analysed using Kaplan–Meier curves.

A total of 45 cases, male predominance (55.6%), and mean age at diagnosis of 6.5 years. Dilated and hypertrophic cardiomyopathy were the most common (33.3%). The main reason for diagnosis was the investigation of cardiovascular symptoms (60.9%). Genetic investigation occurred in 66.6%, a positivity rate of 60%. Multi-organ/system involvement was significantly associated with a positive genetic result (77.7%, p = 0.017). Mortality was 11.1%; survival was significantly lower in the following conditions: ejection fraction < 30% (p < 0.0001), functional class III/IV (p < 0.0001), heart failure (p = 0.0091), use of three or more cardiovascular medications (p < 0.001), N-Terminal Pro-B-Type natriuretic peptide >1000pg/mL (p = 0.004), and heart transplant indication (p < 0.001).

These findings provide novel data in Brazil, highlight a high rate of positive genetic test, particularly among patients with systemic involvement and identify key clinical predictors of mortality to guide risk stratification and care.