The composition of macro- and micronutrients in milk from six patients with tightly controlled insulin-dependent diabetes mellitus [median glycosylated hemoglobin concentrations at parturition of 5.2% (range 4.9-5.3%, reference range 4.9-6.6%) and 6 wk thereafter of 6.1% (range 5.0-6.3%, reference range 5.0-6.4%) was compared with that from five control subjects. Milk samples were collected halfway through a single breast-feeding at days 3-5 (colostrum); 7, 9, and 10 (transitional milk); and 12, 15, 17, 21, 25, 29, and 35 (mature milk). We found no abnormalities in macronutrient (triglycerides, lactose, and protein), cholesterol, glucose, and myoinositol concentrations or fatty acid composition. Two of three longitudinally studied patients showed rather constant ratios between glucose concentrations in milk and capillary blood. The present data suggest that tight control corrects a multitude of milk abnormalities associated with moderate and poor control.