Twenty-four pregnancies at risk for Hurler disease (MPS I) were monitored by measurement of alpha-iduronidase in chorionic villi. Adequate samples were obtained for direct assay of the villi in 22 pregnancies. Five were found to be affected and the pregnancies were terminated. In another pregnancy an equivocal result was obtained on direct assay but analysis of the cultured chorionic cells showed the fetus to be affected. In one pregnancy where an exceptionally small biopsy was obtained, direct assay indicated the fetus to be unaffected. Following amniocentesis this result was shown to be incorrect. These results confirm that, provided an adequate sample is obtained, an accurate diagnosis can be made by direct assay of chorionic villi in pregnancies at risk for Hurler disease.