Whether the magnitude and distribution of left ventricular hypertrophy in patients with hypertrophic cardiomyopathy are established at birth, or whether they evolve during the first years of life, is unknown. Accordingly, we conducted serial echocardiographic studies in 39 children with a family history or morphologic evidence of hypertrophic cardiomyopathy. The patients were initially evaluated at 4 to 15 years of age (mean, 11) and most recently at 9 to 20 years (mean, 16). During a follow-up period of 2.5 to 6.8 years (mean, 4), the magnitude and extent of preexisting left ventricular hypertrophy markedly increased in 17 patients and the morphologic appearance of the heart evolved from normal to hypertrophic in 5 others. In these 22 patients the left ventricular wall thickness increased strikingly (by 6 to 23 mm, a change of 101 +/- 62 percent); these increases significantly exceeded those expected as a consequence of normal growth (13 +/- 10 percent; P less than 0.001) and were not associated with symptomatic deterioration or related to subaortic obstruction. We conclude that left ventricular hypertrophy may develop or progress spontaneously in patients with hypertrophic cardiomyopathy during childhood, when body growth is considerable. Since echocardiograms may be normal during childhood--before the morphologic features of hypertrophic cardiomyopathy develop--a single echocardiographic examination of young relatives of patients with hypertrophic cardiomyopathy may not exclude this disease.