Three males with a 46,XX karyotype are described. In two of them, evidence of a Y-containing line was found. In the first case, 1 of 500 lymphocyte metaphases was 47,XXY. In the second, 1 of 400 oral mucosa cells contained a Y body. The proportion of low-grade XX/XXY mosaics found among XX males now stands at about 17%. © 1978 Springer-Verlag.