Data on handedness and cognitive performance in an adolescent sample of same-sex twins were collected, and questions about incidence of left-handedness in twins and the relation between handedness and cognitive performance were considered. Same-sex twins have been found to have a higher incidence of left-handedness than that usually reported in the general population. There is a high incidence of handedness discordance (one twin right-handed and his cotwin left-handed) in both monozygotic and dizygotic twin pairs. In this sample, males and Blacks had particularly high rates of left-handedness. Among the monozygotic and dizygotic discordant pairs, quite different patterns of cognitive performance were found. On the Raven Progressive Matrices, the Columbia Test of Mental Maturity, and the Peabody Picture Vocabulary Test, the left-handers did better than their right-handed cotwins in the monozygotic pairs and the right-handers did better than their left-handed cotwins in the dizygotic pairs. Within subjects, no signficant differences were found for superiority of spatial or verbal abilities for either right-handers or left-handers. No general statements can be made about the performance of left-handed twins, since performance differs according to zygosity and handedness of cotwin configurations.