To study the question of whether one brand of oral contraceptives may be as acceptable as another for use of publicly-assisted family planning programs, a double blind study of two well-known brands, Ovral and Norinyl, was undertaken in Costa Rica and Trinidad. The pills were randomly assigned to 1,200 women. Common side effects - nausea, dizziness, vomiting, headaches - were associated with both Norinyl and Ovral. Differences in event rates for these conditions were much more marked by country than by the pill used. Ovral was associated with increases in skin problems, notably chloasma, in Cost Rica. A higher percentage of women using Norinyl reported intermenstrual bleeding and spotting in both countries. In Costa Rica continuation rates for Norinyl were adversely affected by this. With these exceptions there appear to be no important differences between the brands that would affect their use in family planning programs.