An assessment of sex-related differences in mathematics achievement and related attitude variables of 13-year-old students from 20 different countries is presented in this study. The analyses of student achievement were based on pretest and posttest data from seven of the participating countries and a single-test condition from the remaining l3 countries. Data were obtained from items which addressed the areas of arithmetic, algebra, geometry, measurement, and statistics. An extensive questionnaire, which also contained five subscales, was used to measure student attitudes toward mathematics. The subscales focused on: (l) mathematics as process; (2) home support; (3) mathematics and society; (4) mathematics and myself; and (5) gender stereotyping. The pattern of achievement results indicated that girls were more successful than boys in Belgium, Thailand, Finland and Hungary but least so in France, Nigeria, Israel, and the Netherlands. With regard to attitude measures, the largest differences between countries and sexes occurred for the scale, gender stereotyping. Summaries are provided in table and graph form. (ML)