447 women were treated with either norethindrone, norethindrone acetate or norethindrone with ethinyl estradiol to prevent conception. The total length of time patients remained under treatment varied from 1 to 60 months for an over-all total experience of 8308 cycles. Only 1 woman conceived, and she had not followed instructions. 4 others were pregnant when treatment was instituted, as learned by hindsight. 58 women conceived within 1 to 12 months after they discontinued medication taken previously for periods of time ranging from 1 to 31 months. Most conceived within 3 months. None of the women suffered thrombophelbitis. Breakthrough uterine bleeding was more common with the administration of norethindrone or norethindrone acetate than with norethindrone acetate combined with ethinyl estradiol. Nausea, retention of fluid, and mastalgia were more common with the progestin-estrogen combination. Ophthalmologic examinations, 17-ketosteroid studies, follicle-stimulating hormone analysis, and blood cholesterol determinations on patients selected at random and after varying periods of treatment were within normal limits.