BACKGROUND Hypertrophic cardiomyopathy (HCM) is associated with mutations of genes coding for major sarcomeric proteins, but the mechanism of hypertrophy is unknown. As hypertrophy may not develop until adolescence, an altered response to physiological growth stimuli may regulate the hypertrophy process. OBJECTIVE This study examined the relationship between age and changes in left ventricular (LV) wall thickness in patients with HCM. METHODS Forty-three patients who had definite electrocardiographic and echocardiographic evidence of HCM were studied with serial 2D and M-mode echocardiograms at least two years apart (mean interval 5.5 +/- 3.0 years). LV cavity dimensions, septal and posterior wall thicknesses, and LV mass indices were compared with data from an age- and gender-matched control group. RESULTS In patients with HCM aged ten to 20 years (n = 9), there was an increase in septal wall thickness during the study period from 15.9 +/- 6.2 mm to 19.3 +/- 2.1 mm (p < 0.01). This increase (3.4 +/- 2.5 mm) greatly exceeded the change in septal thickness observed in the control group between the ages of ten and 20 years (0.8 +/- 0.3 mm, p < 0.01). There was a lesser increase in posterior wall thickness from 9.8 +/- 2.1 mm to 11.5 +/- 3.5 mm (p = 0.07). In patients with HCM aged 21-40 years (n = 11), there was also an increase in septal wall thickness during the study period from 16.0 +/- 2.2 mm to 17.8 +/- 3.0 mm (p < 0.05), but no change in septal thickness in the control group. In contrast, the patients aged > 40 years (n = 23) showed no significant change in either septal or posterior wall thickness during the study period. LV mass index increased in the ten to 20 years age group from 128 +/- 24 g/m2 to 164 +/- 20 g/m2 (p = 0.01), but this increase was not observed in the older age groups. CONCLUSIONS LV hypertrophy is progressive, particularly in the septum, during adolescence and early adult life in patients with HCM. As progressive hypertrophy may continue after somatic growth has ceased, an abnormal myocardial response to physiological growth regulators is less likely to be the principal stimulus to hypertrophy. Gene-gene interactions, changes in haemodynamic load or environmental factors may modulate the development of hypertrophy. Serial measurements of ventricular wall thickness in the first two decades of life, and probably until the fourth decade of life are advisable in patients suspected of having HCM.