A method is described for quantification of 7-dehydrocholesterol (7DHC) and other neutral sterols by gas chromatography/mass spectrometry for diagnosis of Smith-Lemli-Opitz syndrome, an apparent primary defect of cholesterol biosynthesis associated with low plasma levels of cholesterol and high levels of its precursor, 7DHC. Results are summarized for specimens from normal controls and from 40 patients with Smith-Lemli-Opitz syndrome (SLOS). Whereas the concentration of 7DHC (as a combined peak of 7DHC and iso-7DHC) in normal infant plasma was found to be 0.10 +/- 0.05 (S.D.) microgram/ml, the level in patients with SLOS ranged from 2.7 to 470 micrograms/ml, or from 10 to more than 2000 times the upper limit of normal. Patients with milder type I SLOS as well as those with the more severe type II SLOS were found to have the same sterol abnormality. Although most infants with SLOS had plasma cholesterol levels lower than 400 micrograms/ml (40 mg/dl), several older children with only mildly increased levels of 7DHC had normal plasma cholesterol levels. Diagnostically useful, comparably increased levels of 7DHC were found in amniotic fluid and cultured skin fibroblasts from patients with SLOS. More mildly increased levels of 7DHC were also found in both plasma and cultured skin fibroblasts of SLOS heterozygotes. The method described uses capillary columns and GC/MS instrumentation available in most biochemical genetics laboratories and should prove useful not only for diagnosis and prenatal diagnosis of Smith-Lemli-Opitz syndrome, but also for identification of other possible inborn errors of sterol biosynthesis.