Congenital adrenal hyperplasia (CAH) is a group of autosomal recessive disorders mainly due to defects in the steroid 21-hydroxylase (CYP21) gene. To determine the mutational spectrum in the Tunisian CAH population, the CYP21 active gene was analyzed in 51 unrelated patients using our cascade strategy (digestion by restriction enzyme, sequencing). All patients had a classical form of 21-hydroxylase deficiency. Mutations were detected in over 94% of the chromosomes examined. The most frequent mutation in the Tunisian CAH population was found to be Q318X, with large prevalence (35.3%), in contrast to 0.5-13.8% described in other series. Incidence of other mutations does not differ, as previously described: large deletions (19.6%), mutation in intron 2 (17.6%), and I172N (10.8%). Four novel mutations were found in four patients with the salt-wasting form. These four novel mutations include three point mutations that have not been reported to occur in the CYP21P pseudogene: R483W, W19X, 2669insC, and one small conversion of DNA sequence from exon 5 to exon 8. Our results have shown a good genotype/phenotype correlation in the case of most mutations. This is the first report of screening for mutations of 21-hydroxylase gene in the Tunisian population and even in the Arab population.