A French multicentre study of hypertrophic cardiomyopathy has recruited 260 subjects belonging to 18 families. At least 3 persons from each family included had a hypertrophic cardiomyopathy. A Doppler echocardiographic examination was performed in all members of these 18 families. The diagnosis of hypertrophic cardiomyopathy was based on M mode and/or 2D observations of parietal hypertrophy: wall thickness over 13 mm. Asymmetrical forms were distinguished from symmetrical forms by a septum/posterior wall ratio of over 1.3 in M mode or 2D study. The patients had an average age of 40 +/- 18 years, 127 were men and 133 women. One hundred and sixty eight were considered to be normal, 87 had hypertrophic cardiomyopathy and 5 were border line. The maximal diastolic wall thickness on M mode recording of pathological cases was 19.2 +/- 4.8 mm compared with 9.5 +/- 1.9 mm in healthy subjects (p < 0.001). The septum/posterior wall ratio was 1.8 +/- 0.62 in pathological cases and 1.1 +/- 0.8 in normal ones (p < 0.001). Eighty five per cent of the cardiomyopathic cases were asymmetrical (74/87) and only 15% were considered to be symmetrical (13/87) on M mode study whereas 5% were concentric on 2D echocardiography. By Maron's classification, type I hypertrophy was observed in 6% of cases (4/64), type II in 30% (19/64), type III in 58% (37/64), and type IV in 6% (4/64). Signs of obstruction were looked for and systolic anterior motion of the mitral valve was observed in 52% of pathological cases (45/86) and mid-systolic aortic valve closure in 30% (25/83).(ABSTRACT TRUNCATED AT 250 WORDS)