Nuclear sex determinations were done on 96 patients (82 boys and 14 girls) under psychiatric care for the hyperactive child syndrome. Complete karyotype analysis was donc on lymphocytes from peripheral blood of 23 of these patients (20 boys and 3 girls). No evidence of sex chromosome aneuploidy or of other chromosome abnormality was found. It was concluded that a recognizable chromosome abnormality is not a major cause of the hyperactive child syndrome.