In order to investigate the pathological basis of muscle hypertrophy in Duchenne dystrophy, 9 biopsy specimens of the lateral gastrocnemius and 7 of the vastus lateralis were compared. All patients had calf hypertrophy and normal strength in gastrocnemius-soleus, whereas the quadriceps biopsied were all atrophied and weak. The patients' ages ranged from 4 to 11 years. The pathological and histochemical changes were assessed semi-quantitatively. Comparison of the gastrocnemius and quadriceps groups showed that the number of hypercontracted fibres, the degree of endomysial fibrosis and the degree of fat infiltration were significantly higher in the quadriceps. The fibre type differentiation was better in the gastrocnemius group. The mean fibre diameter was above normal in all gastrocnemius biopsies and showed no increase with age. In the quadriceps, fibre hypertrophy was found early in the disease but had changed into fibre atrophy in the three oldest patients. When present, fibre hypertrophy involved both fibre types. The amount of fat-fibrosis per unit area was increased in both groups, but more severely so in the quadriceps. These results indicate that there is no true muscle hypertrophy in the gastrocnemius, in which the fat-fibrosis component was increased in all patients and that the dystrophic process is more active in the quadriceps. The finding of persistent fibre hypertrophy in the gastrocnemius is discussed with respect to the postural abnormalities observed in the lower limbs in Duchenne dystrophy.