Screening for fetal chromosomal abnormalities on the basis of maternal age has not resulted in a substantial fall in the proportion of infants born with an abnormal karyotype. Most fetuses with major chromosomal abnormalities have defects that can be recognised on detailed ultrasonographic examination. Therefore, provided the cardinal signs of each chromosomal syndrome are recognised, it is possible that screening by ultrasound examination could have a greater impact. We karyotyped 2086 fetuses after ultrasonographic examination had revealed fetal malformations, growth retardation, or both. Chromosomal abnormalities were detected in 301 (14%) cases and were more common among fetuses with multisystem malformations (29%) than among those with isolated defects (2%). The commonest chromosomal abnormality was trisomy 18, followed by trisomy 21, triploidy, Turner's syndrome, unbalanced chromosomal rearrangements, and trisomy 13. Trisomy 18 was associated with strawberry-shaped head, choroid plexus cysts, facial cleft, micrognathia, heart defects, exomphalos, malformations of hands and feet, and growth retardation. In trisomy 21, the associated defects were subtle and included nuchal oedema, macroglossia, atrioventricular septal defects, mild hydronephrosis, clinodactyly, and sandal gap. The frequency of autosomal abnormalities increased with maternal age, but if fetal karyotyping had been restricted to mothers older than 35 years, large proportions of chromosomally abnormal fetuses would not have been diagnosed prenatally (64-97%). Our findings provide guidelines as to which defects to search for in screening studies for the detection of chromosomal abnormalities.