A man charged with a sexual offence was found to have complex karyotype anomaly, including a Y-chromosome of aberrant structure. Other features were: very small stature, skeletal deformities and obscure neurological defects. There were no gross psychiatric symptoms, nor was there mental retardation. Q- and G-banding and photometric scanning of the chromosomes of the propositus and his three healthy brothers indicated that the aberrant Y-chromosome probably arose from a reciprocal translocation, which may be written: t(Y: Y) (qter leads to p11: : q11 leads to qter). Fibroblast cultures differed from the lymphocytes: in the former the majority of the cells lacked the abnormal Y-chromosome, their karyotype being 45,X0.