One hundred and seventy-five women took part in a comparative clinical trial of four progestogen-only oral contraceptives and were followed for either a year or until treatment was discontinued. Megestrol acetate 0.25 mg. was found to be a very ineffective contraceptive, 21 out of 43 women becoming pregnant. One, three, and four pregnancies occurred during treatment with norethisterone acetate 0.3 mg., norgestrel 0.05 mg., and chlormadinone 0.5 mg., respectively, corresponding to pregnancy rates of 4, 9, and 12 per 100 woman-years of use.All three effective progestogens were very much less acceptable than modern low-dose combined oral contraceptives. Discontinuation of treatment for medical reasons (particularly menstrual disturbances) during the course of only one year affected 24% receiving norethisterone acetate, 38% receiving norgestrel, and 46% receiving chlormadinone.