To determine the occurrence of familial and sporadic forms of hypertrophic cardiomyopathy (HC) 74 first-degree relatives of 21 patients with proven HC were studied by M-mode and two-dimensional echocardiography. A diagnosis of HC was made in 11 relatives (15%) while it was excluded in 61 of them (82%); 2 subjects (3%) were considered neither affected nor unaffected (borderline left ventricular hypertrophy suggestive of HC). Inspection of pedigrees revealed 38% of familial forms of HC with an autosomal dominant pattern of inheritance in 5/8 families (62%). Furthermore, among those relatives judged unaffected by means of full echocardiographic criteria for HC, an attempt was made to find out whether minor changes of left ventricular geometry were present for their possible implications in genetics of HC (latent or potential forms, low phenotypic expression of the disease). Eleven out of 61 unaffected relatives had a left ventricular wall thickness radius ratio > .50 (equivocal hypertrophy), a value that was higher than two standard deviations of the control group. Assessment of clinical significance of borderline and equivocal hypertrophy in relatives of patients with HC is required for a better understanding of genetic transmission of this disease. In this view the occurrence of sporadic and familial forms of HC might be revisited.