The second most common cause of congenital adrenal hyperplasia is 11 beta-hydroxylase deficiency, an autosomal recessive disorder. We performed genetic analysis of CYP11B1, the gene encoding steroid 11 beta-hydroxylase, in three patients with classic 11 beta-hydroxylase deficiency. Herein we describe the first splice donor site mutation, a new nonsense mutation, and a new missense mutation in this disorder. An African-American patient was found to be a compound heterozygote for a codon 318 + 1G --> A substitution at the 5'-splice donor site of intron 5, in combination with Q356X, a nonsense mutation previously reported in an African-American patient. A Caucasian patient was found to be a compound heterozygote with a novel missense mutation, T318R, in combination with a previously reported 28-bp deletion in exon 2. A different mutation at codon 318 (T318M) has been described previously. A Caucasian patient was heterozygous for a novel nonsense mutation (Q19X) in exon 2. The second mutation was not identified in this patient. Multiple apparent polymorphisms were also observed. Two of these polymorphisms in CYP11B1 represent sequences from CYP11B2, suggesting that gene conversion may have occurred. In summary, we have identified three novel mutations and two previously reported mutations in CYP11B1 patients with 11 beta-hydroxylase deficiency. Our data suggest the presence of a mutational hot spot at codon 318 of CYP11B1, and the possibility of a founder effect in frequently identified mutations.