The etiology of diabetic nephropathy is currently unknown, but blood pressure is believed an important risk factor. A total of 173 subjects with type I diabetes who participated in two or more exercise tests with simultaneous measurements of blood pressure (BP) and heart rate were studied. Subjects with diabetes had significantly elevated resting and exercise diastolic and systolic BPs when compared with controls. This was not related to the presence of albuminuria. During exercise, elevation of diastolic BP was present in 65 (38%) of the 173 test subjects. Elevated exercise diastolic BPs among the subjects with diabetes (excluding controls) was present only for subjects with elevated exercise and overnight albuminuria. It is concluded that factors other than early diabetic nephropathy are likely related to elevated resting and exercise BPs in subjects with diabetes compared with controls. Early renal damage, however, is related to further elevations in resting and exercise diastolic BPs within the group of subjects with diabetes.