In this study we report the physical and psychological findings in 4 males with 48,XXYY syndrome. They were diagnosed at the ages of 4, 6, 18 and 25 years respectively. The major indication for chromosomal analysis in these four slightly mentally retarded males was not their clinical appearance but the presence of behavioral problems with personality disturbances i.e. psychotic reactions, loss of structure, violent and impulsive reactions. These data are compared with previous findings in the literature.