Hemophilia A (HA), a common inherited bleeding disorder in humans, is due to the deficiency or absence of the factor VIII (FVIII) activity. The cloning of the FVIII gene has made molecular probes available for the characterization of the basic defect in this disease. In this study we describe six different mutations in the FVIII gene detected by DNA analysis of 100 HA patients of Italian descent. In two of them, with a severe clinical picture, we identified two novel deletions, one in the middle of the FVIII gene from exons 7 to 22 and the other encompassing the entire factor VIII gene. Both of these patients produced antibodies to factor VIII. In a patient with mild HA we detected a duplication of exon 13, which is a rearrangement not yet described within the FVIII gene. A possible explanation for the mild phenotype in this patient is that the molecular defect results in the production of an unstable FVIII protein with residual 10% FVIII activity. Screening by Taq I restriction endonuclease detected three mutations that were further characterized by direct sequencing on amplified DNA: a C-T substitution at codon 1960, in exon 18, converting the codon for arginine to a non- sense codon; and a G-A substitution at codon 2228 and 2326, in exons 24 and 26 respectively, resulting in the substitution of glutamine for arginine. All three of these mutations have been previously described. The non-sense mutation and the codon 2228 G-A mutation was found in patients with severe HA, while the codon 2326 G-A mutation was associated with a quite severe condition. These results confirm that the molecular bases of HA are very heterogeneous and provide further evidence that recurrent mutations are not uncommon in this system.