A total of 55 patients with hypertropic cardiomyopathy (HCMP) with and without circulatory insufficiency were examined at rest and in the restorative period (up to 24 hours) after graded exercise. The serum levels of lactate were measured and the activity of lysosomal enzymes: acid phosphatase, beta-glucosidase and beta-galactosidase were assayed. Healthy untrained donors (n = 21) and athletes (n = 12) with moderately working myocardial hypertrophy, as evidenced by echocardiography served as controls. The patients with HCMP had higher lactate levels and enhanced activity of lysosomal enzymes both at rest and during exercise than the controls. Highest hyperenzymia was found in HCMP patients with circulatory insufficiency and in those without the latter who showed progressive disease even with signs of circulatory insufficiency during 6-9 month follow-ups. In these groups, the patients had the lowest capacity to restore after exercise. It is concluded that the high activity of blood lysosomal enzymes in HCMP patients is a poor predictor.