Smith-Lemli-Opitz syndrome (SLOS) is a multiple congenital anomalies/mental retardation disorder possibly due to a defect of delta 7-sterol reductase, leading to low plasma cholesterol levels and to the accumulation of 7-dehydrocholesterol (7-DHC) and other cholesterol precursors. This study aimed to identify clinical features that could potentially be specific indicators for the clinical diagnosis of SLOS, and to test the reliability of ultraviolet spectrophotometry (UVS) as a biochemical screening procedure for the syndrome. Twenty patients with clinical suspicion of SLOS, referred to 11 Italian paediatric and clinical genetic centres, were collected during 1994. In 10 patients the diagnosis was confirmed biochemically by gas chromatography/mass spectrometry (GC/MS) analysis of serum sterols, whereas in the other 10 patients the serum sterol profiles were normal. A comparison between confirmed SLOS patients and biochemically negative subjects did not show clinical signs specific for the syndrome. UVS measurement of 7-DHC correlated well with GC/MS profiles, showing 100% sensitivity and specificity. Four out of five patients had serum bile acid concentrations below the normal range of controls.