Experience with prenatal karyotyping of 237 fetuses with sonographic evidence of malformation is reported. Abnormal karyotype was found in 40 cases (16.8 per cent): chromosomal aberrations were found in 19 of the 178 fetuses with an isolated structural anomaly (10.6 per cent) and in 21 of the 59 fetuses with multiple malformations (35.6 per cent). Detailed cytogenetic and morphological information concerning fetuses affected by omphalocele, duodenal atresia, hydrocephalus, multicystic kidney, unilateral hydronephrosis and cystic hygroma is reported. The need for a very careful ultrasound evaluation of fetal anatomy in these pregnancies is stressed, as the risk of a chromosomal anomaly depends mainly on the existence of more than one ultrasonically diagnosed structural defect.