Eighty healthy single births born at a gestational age of 259-294 days were studied in an open, non-randomized, group comparative fashion. The mothers were on average 6 weeks postpartum, healthy, and fully breastfeeding at the start of treatment. Forty-two mothers elected to use the etonogestrel-releasing implant, Implanon, while 38 chose use of a non-hormone medicated intrauterine device (IUD). One month after implant placement, the dose of etonogestrel ingested by the infants via breast milk was 19.86 ng/kg/day, which decreased to 10.45 ng/kg/day at the end of the study period (month 4). The volume of breast milk production was not affected by the use of Implanon. There were no significant differences between groups in milk content of total fat, total protein, and lactose. The timing and quantity of supplementary feedings did not differ between the two groups. Growth of the infants was analyzed by treatment and gender. For the girls, no differences between groups were seen for body weight, body length, and head circumference. The same applied to the boys except for a somewhat larger, although not statistically significant, increase in body weight for boys whose mother used Implanon. There was a low incidence of intercurrent illnesses in the infants of both groups. None of the conditions was of a serious nature. From the present study, we conclude that Implanon did not change the volume and composition of breast milk. The low concentration of etonogestrel ingested by the infant was not associated with adverse effects.