Diabetes may affect the secretion of prolactin, the principal lactogenic hormone. Because adequate amounts are critical to the establishment of lactation, we assessed the prolactin status of 33 women with insulin-dependent diabetes mellitus (IDDM), 33 women without diabetes, and 11 reference women participating in a study of lactation from 2 to 84 d postpartum. Circulating concentrations of serum prolactin declined temporally for all women and did not differ significantly among any of the groups. During the first postnatal week, milk immunoreactive prolactin concentrations were lower for women with IDDM than for control and reference women and the inverse relationship between lactose and milk prolactin, which was significant at day 2 postpartum for reference women, was delayed until day 14 postpartum for women with IDDM. Early breast-feeding activity, increased breast-feeding frequency, and good glycemic control enhance prolactin secretion and should be promoted during lactation in women with IDDM.