Smith-Lemli-Opitz syndrome (SLOS) is an autosomal recessive MCA-MR disorder caused by mutations within the 7-dehydrocholesterol reductase gene, DHCR7. The diagnosis is based on the biochemical findings of elevated plasma 7-dehydrocholesterol (7DHC) levels. It is a panethnic condition with variable mutation frequencies in different populations. Ten Cuban patients and four Canadian patients of Mediterranean ancestry with SLOS are reported herein. All these patients are at the mild end of the clinical spectrum (the highest Kelley-Hennekam severity score was 28 in one patient). All patients had genotypes which were compound heterozygous or homozygous for T93M; in all the Mediterranean patients the T93M mutation appeared to be associated with the J haplotype. Another compound heterozygote for T93M was of Ukrainian/Irish ancestry; in this patient the T93M was associated with a new haplotype designated K. The T93M mutation was initially reported as the most common in a series of patients from Italy. Our identification of a single haplotype associated with the T93M mutation in patients whose ancestors originate in the region of the Mediterranean Sea basin suggests a founder effect.