This study examined the temporal relationship between attitude toward mathematics and mathematics achievement among first-year secondary (i.e. middle) school students (grade 7, about 14-15 years) with a focus on sex differences in Central Uganda. Random-intercept cross-lagged panel models, based on structural equation modeling, were used to analyze data which were collected through students' questionnaires and mathematics tests at three measurement points. The proportion of variance at the class and school level was larger for the achievement measures than for the attitude measures. At these two levels, also the correlation coefficients between the two constructs were higher for boys than for girls. The study found evidence in support of the reciprocal-effects model as the best-fitting structural model within and across both sexes. Between the first and second measurement points, there was evidence for the self-enhancement model (attitude influencing achievement), while between the second and third points, the skill-development model (achievement influencing attitude) was supported.