The present study assessed the longstanding question of whether there is a relationship between sex hormones and cognitive ability in young adolescent girls and boys. Three “theories” of the relationship between hormone levels and cognition were identi” ed from a broad literature base and were used to formulate hypotheses regarding the relationship between sex hormones and cognitive ability in early adolescents. Fifty-six boys, aged 10-14, and 52 girls, aged 9-14, completed two standardised spatial ability tests and one standardised test of verbal ability. Three blood samples, at 20-minute intervals, were obtained to determine levels of estradiol and testosterone. Data were collected on three occasions for all measures at 6-month intervals. A positive linear relationship between testosterone and spatial ability was identi” ed for boys at each time of measurement for both measures of spatial ability (i.e., mental rotation and block design). For girls, a positive linear relationship was identi” ed between testosterone and mental rotation only at time 3. In addition, longitudinal relationships were identi” ed between change in testosterone levels and change in spatial ability for both girls and boys. The conclusion was that testosterone is related to cognitive functioning during pubertal development for boys and to a lesser extent for girls.