Smith-Lemli-Opitz syndrome (SLO) is an autosomal recessive disorder characterised by craniofacial dysmorphism, mental retardation, multiple congenital anomalies, and increased levels of 7-dehydrocholesterol (7-DHC) in body tissues and fluids. SLO is caused by mutations in the DHCR7 gene which encodes 7-dehydrocholesterol reductase, the last enzyme of cholesterol biosynthesis pathway. In our investigation, we screened 682 dysmorphic/mentally retarded Portuguese patients for abnormal levels of 7-DHC in blood by UV spectrometry. We identified six unrelated patients with SLO (0.87% of total). Mutational analysis of the DHCR7 gene led to the identification of seven distinct mutations, three of which are new (F174S, H301R, and Q98X). The common IVS8-1G > C and T93M variants together with the H301R accounted for 70% of the all SLO alleles in our population. Our findings contribute to the variegate array of pathological changes in the DHCR7 gene among different European populations.