The RSH/Smith-Lemli-Opitz syndrome (RSH/SLOS) is an autosomal recessive malformation syndrome comprising microcephaly, developmental and growth retardation, characteristic facial anomalies, midline cleft palate, and genital and limb anomalies. Recently, biochemical evidence of an inborn error of cholesterol biosynthesis at the level of 7-dehydrocholesterol (7DHC) reductase was reported in children and adults with RSH/SLOS. We report on two sibs with a variant form of RSH/SLOS whose sterol metabolism in cultured lymphoblasts is abnormal but differs from that of patients with classical RSH/SLOS. The children have relatively mild physical and developmental abnormalities, but a phenotype still consistent with the diagnosis of RSH/SLOS. Their plasma cholesterol levels are only mildly depressed, and they have less markedly increased plasma levels of 7DHC than most patients with classical RSH/SLOS. Cultured lymphoblasts from our patients accumulated 7DHC to the same degree as classical RSH/SLOS lymphoblast when grown with cholesterol-depleted fetal calf serum. However, unlike other RSH/SLOS cells, the increase in cellular 7DHC levels was not suppressed when the cells were grown in the presence of cholesterol from untreated fetal calf serum. The parents' sterol metabolism was also strikingly abnormal in that the levels of 7DHC in their lymphoblasts were markedly elevated compared with those of lymphoblasts from other RSH/SLOS parents. Our findings suggest that these mildly affected RSH/SLOS sibs may have a genetic disorder of sterol metabolism that is related to but biochemically different from classical RSH/SLOS, possibly one affecting intracellular transport of sterols.