Heart muscle disease in acromegaly manifests usually as cardiac hypertrophy. Based on a retrospective analysis, it was suggested that cardiac hypertrophy is slowly reversible after normalization of plasma growth hormone levels. The reversibility of acromegalic heart muscle disease during and after treatment of acromegaly was studied prospectively. A cohort of 78 patients was examined echocardiographically in 1981, and 38 survivors of this group were reexamined 10 years later. Patients were classified according to original hormonal activity in 1981, and change in hormonal activity during follow-up into the following 4 groups: group I--hormonally inactive for entire follow-up (n = 10); group II--hormonally active for entire follow-up (n = 11); group III--initially hormonally inactive with later resurgence (n = 6); and group IV--initially hormonally active with later normalization of growth hormone levels (n = 11). No significant echocardiographic changes occurred during follow-up in group I. Left ventricular posterior wall and septal diastolic thickness, and left ventricular mass increased significantly (all p < 0.05) in group II. Left ventricular posterior wall thickness, mass and diastolic volume increased significantly (p < 0.05, < 0.01 and < 0.001, respectively) in group III. On the contrary, there were significant decreases in left ventricular mass, and both diastolic and systolic left ventricular volumes (p < 0.01, < 0.05 and < 0.05, respectively) in group IV. It is concluded that both hypertrophy and dilatation of the left ventricle in acromegaly are slowly reversible after successful treatment. On the contrary, continuing or relapsed hyperproduction of growth hormone causes further deterioration of acromegalic heart disease.