We report a novel missense mutation of CYP11B1 causing non-classical 11β-hydroxylase deficiency in 3 members of a consanguineous Turkish family. Two siblings presented with clinical evidence of precocious pseudopubarche. Biochemistry suggested 11β-hydroxylase deficiency and genetic analysis revealed that they were homozygous for the missense mutation L489S within exon 9 of the CYP11B1 gene. The unaffected parents were heterozygotes for the same mutation. In addition, a paternal aunt of the affected siblings presenting with primary infertility and mild hirsutism was found to have the same homozygous mutation. This is the first report of a homozygous mutation in non-classical congenital adrenal hyperplasia that cosegregates with clinical phenotype. The significance of the missense mutation L489S in CYP11B1 is further supported by the conservation of leucine at position 489 in CYP11 genes in eleven other species. Molecular modelling of the enzyme suggests that the mutation L489S in CYP11B1 may alter the enzyme’s substrate-binding affinity. These findings suggest that this homozygous mutation affects 11β-hydroxylase function, resulting in the clinical features of non-classical adrenal hyperplasia in this family.