This study was designed to assess the size and distribution of muscle fiber types in patients with severe chronic obstructive pulmonary disease and stable chronic hypoxemia. Brachial biceps biopsies were performed in 8 patients and 12 controls. Histochemistry was used to count and determine the cross-sectional area of the various fiber-types (1, 2a, and 2b). A significant reduction (P < 0.05) in the proportion of type 2a fibers and an increase in the proportion and cross-sectional area of type 2b fibers were seen in hypoxemic patients. These findings suggest an adaptation of the muscle fibers to a low partial pressure of oxygen in arterial blood.