OBJECTIVE: To determine whether muscle fiber composition and capillary density differed between diabetic and nondiabetic subjects. RESEARCH DESIGN AND METHODS: Muscle fiber composition and capillary density were determined in biopsies from women and men with non-insulindependent diabetes mellitus (NIDDM) and compared with those of control subjects matched for gender, age, obesity, and the waist-to-hip ratio, which are all factors known to influence muscle morphology. RESULTS: Patients with NIDDM, as well as control subjects with abdominal obesity and insulin resistance, showed the same abnormalities in muscle morphology, namely, a low percentage of type I fibers, elevated type II (particularly type IIB) fibers, and a low capillary density. These changes correlated closely with insulin concentrations in both diabetic and nondiabetic groups. CONCLUSIONS: Recent information suggests that insulin may regulate myosin synthesis in muscle in the direction of the changes observed. Therefore, it is possible that muscle fiber composition abnormalities in insulin-resistant conditions are secondary to hyperinsulinemia. However, the low capillary density, hypothetically, may contribute to insulin resistance.