Histochemical muscle fibre composition of human costal and crural diaphragm was determined in biopsies sampled during surgical procedures. Two groups were studied: 10 subjects with normal lung function and 8 patients with chronic obstructive respiratory disease. Muscle fibres were classified as type I (slow twitch) or type II (fast twitch) on the basis of their myofibrillar ATPase pH lability. There was no significant difference in fibre proportions between costal diaphragm (COD) and crural diaphragm (CRD) for both groups of subjects. Yet, in the normal group, the diameters of both types of fibres were larger in COD than in CRD. The diameters of both types of fibres of the obstructive patients were significantly decreased in COD as compared to the normals. Moreover, linear correlations between type I and II fibre diameters and VC, FEV1, FEV1/VC, and body weight were found in COD and not in CRD. The differences found in muscle fibre size support the idea that the mechanical respiratory load is different for COD and CRD, COD being the more important force generator. Chronic obstructive respiratory disease causes a significant decrease in COD fibre size but does not affect CRD.