During a 6-year period (1985-1990) blood karyotyping was performed in 682 fetuses with renal defects. There were: 276 fetuses with mild hydronephrosis; 206 with moderate/severe hydronephrosis; 173 with multicystic dysplasia, and 27 with renal agenesis. The overall incidence of chromosomal abnormalities was 12% (trisomies, n = 63; deletions, n = 9; triploidies, n = 5, and sex chromosome aneuploidies, n = 8). There were more than twice as many males than females, but the incidence of chromosomal defects in females was almost double (18%) than in males (10%). Furthermore, compared to the overall maternal age-related risk, the risk for fetal chromosomal abnormalities was three times higher when there was an isolated renal defect and thirty times higher when there were additional malformations. The risk of chromosomal abnormalities was similar for fetuses with unilateral or bilateral involvement, different types of renal defects, urethral or ureteric obstruction, and oligohydramnios or normal/reduced amniotic fluid volume. Nevertheless, the patterns of chromosomal abnormalities, and consequently that of associated malformations, were related to the different types of renal defects.