CYP11B2, the gene coding for steroid 18-hydroxylase (P-450C18), has been recently shown to be the same gene as that for corticosterone methyl oxidase type I and type II (CMO I & II) which were previously postulated to catalyze the final two steps in the biosynthesis of aldosterone in humans. Molecular genetic analysis of CYP11B2 of three patients affected with CMO I deficiency has revealed that deletion of 5 nucleotides occurs exclusively in exon 1, resulting in a frameshift to form a stop codon in the same exon. Thus, P-450C18 is not produced at all due to the mutation, causing a complete lack of aldosterone biosynthesis in the patients. Restriction fragment length polymorphism analysis has demonstrated that the patients are homozygous and the unaffected parent is heterozygous as for the mutation, indicating that CMO I deficiency is inherited in an autosomal recessive manner. These results provide the molecular genetic basis for the characteristic biochemical phenotype of CMO I deficient patients.