Persons with non-insulin-dependent diabetes mellitus (NIDDM), in the absence of complications, have a decreased exercise performance compared with nondiabetic subjects. However, degree of impairment and factors associated with reduced exercise performance have not been fully characterized. Maximal exercise performance using a graded treadmill protocol was assessed in 10 sedentary persons with uncomplicated NIDDM (aged 51 +/- 7) and 10 healthy age- and activity-matched controls. Potential correlates of exercise performance measured included fasting and post-exercise glucose concentrations and fasting insulin concentration, hemoglobin A1C, hematocrit, and whole blood viscosity. At maximal exercise, diabetic persons had a 24% lower maximal walking time and 20% lower maximal VO2 than controls (both P < 0.05), while hemodynamic measures did not differ between groups. During graded exercise, at work loads below the maximal one, the relationship between VO2 and work load was significantly lower in persons with NIDDM than controls by an average of 16%. No correlations were found between peak exercise performance and any of the potential correlates of exercise performance measured. We conclude that persons with NIDDM have an impaired peak exercise performance not associated with degree of glycemic control. The reduced rate of increase in oxygen consumption during increasing submaximal work loads in NIDDM suggests that limitations in oxygen delivery may impair exercise performance in otherwise healthy persons with diabetes.