Congenital adrenal hyperplasia (CAH) due to 21-hydroxylase deficiency is one of the most common autosomal recessive disorders and occurs in its non-classical form in up to 6% of hirsute women. We report on a young woman with the clinical diagnosis of non-classical CAH and a novel, heterozygous missense mutation CTG-->GTG in exon 8, codon 317, of the steroid 21-hydroxylase CYP21B and complete loss of pseudogenes. Protein sequences of closely related P450 cytochromes and a homology-based 3D model of CYP21B were used for further functional analyses. We found that the mutated residue is part of a large cluster of hydrophobic residues. This cluster has three important features: (1) it is located directly next to the binding pocket, in close vicinity of the heme-cofactor, (2) all amino acids of the cluster are directly connected to two important binding regions, and (3) the packing within the cluster is very dense. Due to the tight packing in the cluster and its direct connection to the binding pocket region, any changes induced by the mutation of residue 317 can be expected to lead to structural shifts within the binding pocket and can explain the clinically observed impairment of 21-hydroxylase activity. In conclusion, the novel mutation L317V of the steroid 21-hydroxylase gene is associated with reduced steroid 21-hydroxylase activity probably due to structural shifts within the binding pocket and a mild phenotype of steroid 21-hydroxylase deficiency. In addition, the results support previous findings in which heterozygous CYP21 mutations are associated with symptoms of hyperandrogenism in susceptible individuals.