The frequency of acne appears to be increased in boys and men of the XYY genotype. This report describes severe acne in a retarded man in whom chromosomal analysis with differential banding suggested duplication of the distal portion of the long arm of a number 13 chromosome, a partial trisomy 13. In addition to retardation and seizures, his malformations, which included narrowed temples, ear anomalies, hexadactyly, and hernias, were consistent with those reported previously in patients with partial trisomy for the distal segment of chromosome 13. This patient and one recently reported retarded boy with chronic acne and trisomy 8 mosaicism suggest that the association of acne and chromosomal abnormality may not be limited to Y chromosome excess.