We have characterized the disease-causing mutations in the steroid 21-hydroxylase genes of 127 patients with different clinical forms of congenital adrenal hyperplasia, representing 186 unrelated chromosomes. The gene was completely absent on 29.8% of the chromosomes, and this together with the I2 splice (27.7%), I173N (20.8%), V282L (5.4%), and R357W (3.8%) mutations constitute 87.5% of all affected chromosomes. In total, 15 different sequence aberrations combine to form 19 different disease-causing alleles. The results confirm that genotyping is an efficient means of diagnosing steroid 21-hydroxylase deficiency, although special consideration is needed to resolve genotypes when full families are not available. Clinical presentations of the different combinations of mutations indicate that genotyping is reliable for prediction of clinical outcome in patients with 21-hydroxylase deficiency. It is especially helpful in determining whether in utero treatment of affected females is indicated and in classifying the severity of 21-hydroxylase deficiency in children diagnosed through neonatal screening, before symptoms have appeared.