The adrenal 11 beta-hydroxylase is a mitochondrial P450 enzyme encoded by the CYP11B1 gene, which is situated on chromosome 8q22 in tandem with the gene for aldosterone synthase (CYP11B2). Deficiency of 11 beta-hydroxylase results in the inability to convert 11-deoxycortisol to cortisol and accounts for 5-8% of cases of congenital adrenal hyperplasia. In the following study the CYP11B1 genes from eight individuals with 11 beta-hydroxylase deficiency were screened for mutations using single strand conformation polymorphism (SSCP) analysis. Sequence analysis of variant exons revealed a 28 bp deletion and a 5 bp duplication exon 2 and five missense mutations, G267R, G267D, Q356X, R427H and C494F, distributed throughout the gene. One of these mutations, G267R, and a G to A transversion at the third nucleotide position of codon 318 occur at the +1 position of the splice donor sites. Mutations were neither the result of gene conversion nor nonhomologous recombination between the two closely related CYP11B genes.