In this study 42 pregnant women with type I diabetes and 28 nondiabetic controls were recruited to participate in a postprandial walking exercise program. Exercise patients were instructed to walk 20 minutes (1 mile) after each meal and were divided into two groups: group 1 were normal nondiabetic controls and group 2 were women with type I diabetes. There were two nonexercise comparison groups: group 3, nondiabetic controls, and group 4, women with type I diabetes. Diabetic women were followed weekly in an intensive perinatal program. Glycemic control was assessed by serial hemoglobin A1 concentration measurements, home blood glucose monitoring, 24-hour glucose profiles, and 24-hour quantitative urinary glucose loss. Glycemic control was modestly but not significantly superior in the diabetic exercise group 2 compared with the diabetic nonexercise group 4. Exercise was associated with lower fasting cholesterol and triglyceride values in both controls and diabetic women, with significantly lower fasting plasma triglyceride levels in the diabetic exercise group (p less than 0.02). There were no adverse effects of postprandial walking exercise in mothers or infants.