We investigated a case (I.I.) of the severe form of mucopolysaccharidosis I (Hurler syndrome). Prenatal diagnosis was requested by the parents and the next pregnancy was monitored. We report here a special difficulty arising in this diagnosis due to the low enzyme activity in the mother's cells (10-15 per cent of controls) as well as in amniotic cells and would like to stress the need for studying the index case as well as the parents' enzyme activities in order to be prepared for possible difficulties at prenatal analysis.

At 10 weeks' gestation, chorionic villus biopsy was obtained from a woman at risk for Sanfilippo type C disease. Acetyl‐CoA: α‐glucosaminide N‐acetyltransferase activity was markedly deficient. The pregnancy was terminated at 12 weeks' gestation and follow‐up study on fetal fibroblasts confirmed the diagnosis.

A case of the Hunter syndrome diagnosed prenatally using chorionic villi is presented. Chorion biopsy was performed in the 10th week of pregnancy. The sample was examined for karyotype and for iduronate sulfatase activity. The fetus was male and the enzyme activity reduced to 4 per cent of normal. Termination of pregnancy occurred at the 11th week.