To document the incidence and management of breast feeding among women with insulin-dependent diabetes mellitus (IDDM), 30 IDDM mothers and 30 controls were followed from birth of their infants to 6 weeks postpartum. The researchers interviewed the mothers in the hospital and by telephone at 1, 2, and 6 weeks postpartum. Fifty-three percent of the IDDM women and 57% of the general hospital population intended to breast feed. Because of the need for neonatal observation for hypoglycemia, most IDDM mothers were separated from their infants for the first 2 days. After 1 week, as many IDDM mothers as controls nursed. Initially, no differences in breast-feeding problems were found between the groups, but between 2 and 6 weeks postpartum, two IDDM nursing mothers but none of the controls experienced clinically diagnosed mastitis. A daily diet prescription of 31 kcal per kilogram of maternal body weight was associated with IDDM mothers' ability to sustain lactation, in contrast with 25 kcal per kilogram of maternal weight for IDDM mothers who stopped nursing. Six-week postpartum fasting plasma glucose levels of IDDM mothers who exclusively breast fed were significantly lower (4.6 +/- 2.2 mmol/L [82 +/- 40 mg/dl]) than the glucose levels of the women who stopped nursing (8.1 +/- 2.1 mmol/L [145 +/- 37 mg/dl]) or the glucose levels of the IDDM mothers who chose to bottle feed (6.7 +/- 1.7 mmol/L [120 +/- 30 mg/dl]). It appears that adequate maternal calories, blood sugar control, early breast stimulation, and mastitis monitoring are necessary for optimal lactation outcome in insulin-dependent diabetic women.