Breast milk volume and composition and infant growth were measured at three- and four-week intervals, up to six months, in a multicenter randomized double-blind trial comparing a low-dose combined oral contraceptive (OC) with a progestogen-only OC. A nonrandom group using nonhormonal methods was also studied in the three centers: Szeged, Hungary; Bangkok, Thailand; and Khon Kaen, Thailand. A fourth group, users of depot-medroxyprogesterone acetate (DMPA) was included in the two Thai centers. Altogether, 341 women were recruited into the study. Combined OCs caused a significant decrease in milk output and total energy content as well as widespread changes in milk constituents. In the DMPA group, no significant changes were observed in milk volume, and only minor shifts occurred in milk composition, which varied between centers. No differences were found between the progestogen-only pill and DMPA. No hormonal contraceptive was associated with any significant difference in infant weight or fat fold, nor in the rate of discontinuation for failure to gain weight. This study reiterates the need to avoid combined OCs during the first few weeks or months of lactation. Both norgestrel and DMPA appear to be safe for use in both developing and developed countries, at least when the nutritional status of the mother and infant are adequate, but further research is needed on the safety of these contraceptives in populations with malnutrition.