First trimester prenatal diagnosis was offered to a couple at risk for having a child with I‐cell disease (mucolipidosis II). The prenatal evaluation was based for the first time on examination of N‐acetylglucosamine 1‐phosphotransferase activity, deficiency of which is the primary biochemical defect in both I‐cell disease and pseudo‐Hurler polydystrophy (mucolipidosis III). Heterozygote levels of this enzyme activity were determined in chorionic villi obtained at 9 weeks of gestation, as well as in cultured trophoblasts derived from this specimen, and led to the diagnosis of an unaffected fetus. This procedure has advantages over that based on detection of abnormal intracellular‐extracellular distribution of lysosomal enzyme activities, which is expressed only in homozygotes and fully expressed only in cell cultur