OBJECTIVE: To describe regional incidence, presentation, and outcome of idiopathic (familial) and Noonan syndrome related infant hypertrophic cardiomyopathy (HCM) between 1969 and 1994. DESIGN: Case series. SETTING: Regional cardiac referral unit of the South West Region of England and south Wales, population approximately four million. PATIENTS: 21 cases of idiopathic (or familial) HCM, and eight infants with Noonan syndrome. MAIN OUTCOME MEASURES: Survival and persistence or resolution of symptoms or cardiac hypertrophy. RESULTS: Incidence: eight cases between 1969 and 1982 (idiopathic 6, Noonan 2), 21 cases between 1982 to 1994 (idiopathic 15, Noonan 6). Mode of presentation: cardiac failure, 17 (59%); murmur, 9 (30%); cyanosis, 2 (7%); family history, 1 (7%). Age at presentation: 0-7 days, 16 (55%); 8 days-4 months, 9 (31%); 5-12 months, 4 (14%). Outcome: five deaths (17%), all < 1 year, all from progressive cardiac failure (idiopathic 3, Noonan 2). Four of these five had not received beta blockade. Among the 24 survivors (follow up 1.3-23.2 years, median 5.5 years) hypertrophy had resolved in nine (38%) (idiopathic 8, Noonan 1), was mild and asymptomatic in seven (29%), and was symptomatic or severe in eight (33%). All 10 infants presenting with septal thickness > 1.3 cm have persistent cardiac hypertrophy. CONCLUSIONS: Mortality in infant HCM is much lower than previously reported and resolution is more frequent. This may reflect increased detection of less severe forms in addition to the success of aggressive medical management including beta blockade.