Corticosterone methyl oxidase (CMO) deficiency refers to disorders of aldosterone synthesis due to mutations in the CYP11B2 gene encoding cytochrome P450c11AS, which is the adrenal aldosterone synthase. Type I CMO deficiency is associated with low concentrations of 18OH-corticosterone and aldosterone, due to severe mutations in P450c11AS; while type II CMO deficiency is associated with high concentrations of 18OH-corticosterone and low concentrations of aldosterone, due to less severe mutations of P450c11AS. A single type of mutation, compound homozygosity for R181W and V386A, has been reported as the cause of CMOII deficiency in an inbred population. We now report a patient with a typical clinical and hormonal picture of CMOII deficiency. Direct sequencing of patient and parent DNAs showed that the mother's allele contributed R181W and the deletion/frameshift mutation delta C372, while the father's allele contributed T318M and V386A. These mutants were recreated in cDNA expression vectors singly and in the parental pairs, showing that neither allele contributed any measurable activity. This would suggest the patient should have CMOI deficiency. These studies suggest that other factors besides P450c11AS are involved in the genesis of the distinctive CMOI and CMOII phenotypes.