Chronic obstructive pulmonary disease (COPD) is associated with an increased load on the diaphragm. Chronic loading on skeletal muscles results in metabolic changes and fiber-type shifts. Therefore, we investigated whether the load on the human diaphragm imposed by COPD altered oxidative enzyme activity, glycogenolytic enzyme activity and mitochondrial energy generating capacity and efficiency. Biopsies of the diaphragm from COPD patients and control subjects were obtained and activities of L(+)3-hydroxyacylCoA-dehydrogenase (HADH, marker for beta-oxidation capacity) and phosphorylase (marker for glycogenolytic capacity) were measured spectrophotometrically. Mitochondrial energy generating capacity was measured by spectrophotometrical and radiochemical methods. Fiber-type distribution was determined electrophoretically. We found that HADH activity was increased with increasing severity of COPD (P=0.05). No change in glycogenolytic enzyme activity was observed. The activity of the mitochondrial respiratory chain complexes III and IV and oxidation of pyruvate was increased with increasing airflow obstruction. These results suggest that in COPD the diaphragm adapts to a higher workload by increasing the oxidative capacity and mitochondrial function.