Silastic capsules containing the synthetic progestin ST-1435 was inserted in 282 women of reproductive age who desired long-term contraception. Each woman received a single implant for 6 months' use. After evaluating the experience of the first 45 subjects, replacement capsules were offered to women desirous of continuing the method after the initial 6 months of use. In the first 6-month segment one pregnancy and 1720 woman-months of use were recorded. The total experience, through as many as six segments of use was 3373 woman-months of use and one pregnancy. The Pearl Index is 0.36 per 100 woman-years. The single pregnancy, recorded in the 1st month of the first segment, may represent a conception prior to implant placement. Amenorrhea was the most common side effect reported, with 83% of the women having at least one nonbleeding interval longer than 60 days during the first segment of use.