The serum activities of muscular enzymes are important for the diagnosis and follow-up of muscular diseases. They reflect the degeneration of muscular tissue. The serum enzyme activities (creatine kinase (CK), creatine kinase MB (CKMB), Aldolase (ALD), Lactat-dehydrogenase (LDH), a hydroxy-buty-rat-dehydrogenase (a-HBDH), glutamat-oxalacetat-transaminase (GOT), and glutamat-pyruvat-transaminase (GPT) in Duchenne muscular dystrophy show an increase during the first three years of life, a maximum between the age of 3 and 4 and an asymptotic decline thereafter. Taking the blood samples under non-standardised conditions the coefficients of variation (VK) differed considerably. For a -HBDH the lowest value was obtained (VK = 0.37). The VK-value of ALD was comparatively high (VK = 0.68). The correlation among the enzymes themselves was high and a-HBDH correlated closest to LDH and a-HBDH also to GPT. Thus we conclude that LDH, ALD- and GPT-determinations may be abandoned in Duchenne dystrophy. In Duchenne muscular dystrophy the course of the different enzyme activities can be described by an heuristic mathematical formula (y = Ae-at + bte-ct). With its aid it is possible to calculate the value of the constant c individually for each patient, if at least 4 enzyme values distributed over 3 or more years are available. The evaluation of the enzymes and clinical data of 88 Duchenne patients has shown that the value of constant c is individually correlated to the speed of progression of the disease. This proved most reliable in the case of the CK (r = 0.43 resp. 0.76) and CKMB (r = 0.45). The mean of the constant c in the group of Duchenne patients (n = 88) was -0.29/year, in Beckers dystrophy (n = 7) -0.14/year and in limb girdle dystrophy (n = 12) -0.21/year. In Duchenne muscular dystrophy the calculation of constant c renders it possible to select patients with similar velocity of progression to reinforce the "power of therapeutic studies". The evaluation of pilot studies may be more objective, if the enzyme values actually measured under therapy are compared with those prospectively estimated by mathematical analysis of the course of the enzyme values before treatment.