The objective of this study was to evaluate the contraceptive efficacy and clinical performance of a Nestorone subdermal implant (NES) in the postpartum period. NES (n = 100) and Copper T intrauterine device (T-Cu; n = 100) acceptors initiated contraception at 8 weeks postpartum and were followed at monthly intervals during the first year and at 3-month intervals thereafter. Pregnancy rates, breastfeeding performance, infant growth, bleeding pattern, and side effects were assessed. Blood and milk NES concentration were measured. No pregnancy occurred in 2195 and 2145 woman-months of NES implant and T-Cu use, respectively. No effect of NES on lactation and infant growth and no serious adverse events were observed. Lactational amenorrhea was significantly longer in NES users (353 +/- 20 days) than in T-Cu users (201 +/- 11 days). More NES users (55.8%) experienced prolonged bleedings than did T-Cu users (36.2%). Concentrations of NES in breast milk ranged between 54-135 pmol/liter. The Nestorone implant is a highly effective contraceptive, safe for breastfed infants because the steroid is inactive by the oral route.