The study objectives were to compare the effects of an etonogestrel-releasing implant (Implanon) and a nonmedicated intrauterine device (IUD) on parameters of lactation in breast-feeding women and on the growth of their breast-fed infants over a 3-year period. Healthy lactating women (28-56 days postpartum) chose either the implant (n=42) or the IUD (n=38). Infant growth during a 3-year follow-up period is reported here. Total duration of breast-feeding coinciding with the mothers' treatment was 421.0 and 423.4 days in the Implanon and IUD groups, respectively. There were no differences between the infant groups in terms of body length, biparietal head circumference and body weight. No abnormalities were reported in psychomotor development or during physical examination. No treatment-related side effects were observed in either group. In conclusion, there were no differences in the growth of breast-fed infants of women treated with Implanon or a nonmedicated IUD. Implanon, therefore, appears to be a safe contraceptive option for breast-feeding women and their infants.