A prenatal cytogenetic diagnosis of 48,XYY, + 21 was made at the 20th week of pregnancy of a 38-year-old woman. The pregnancy was terminated shortly after the diagnosis. The fetus had recognizable phenotypic stigmata of trisomy 21. The major abnormalities include broad flat nasal bridge, low-set ears with angulation of the upper helices, flat occiput, strikingly thickened neck with bilateral skin folds, endocardial cushion defect, anomalous origin of the right subclavian artery distal to left subclavian artery, and malrotation of the intestine. The size of the fetus and the placenta indicated intrauterine growth within the expected limits. The interaction between XYY and trisomy 21 on the phenotype of the fetus with this double trisomy could not be determined.