Acute intermittent porphyria (AIP) is an autosomal disorder caused by molecular abnormalities in the gene coding for hydroxymethylbilane synthase (HMBS), the third enzyme in the heme biosynthetic pathway. So far, more than 170 different mutations responsible for AIP have been identified worldwide in the HMBS gene. In this study we have performed molecular characterization in 14 patients with suspected diagnosis of AIP and in 29 family members of Italian ancestry. Molecular analysis of the HMBS gene allowed us to identify 13 different mutations among 14 patients with reduced HMBS activity: 5 splicing defects (IVS9+22 G>A, 612 G>T, IVS11-2 delA, IVS12+2 T>C, and IVS13-1 G>A), 1 small insertion (182 insGA), 1 small deletion (730-731 delCT), and 6 missense/nonsense mutations (76 C>T, 295 G>A, 331 G>A, 580 C>T, 673 C>T, and 874 C>T), resulting in single-amino-acid substitutions or protein truncations. Six of these molecular abnormalities had already been described and 7 are new findings. In a previous work on an Italian population we detected 7 different mutations among 8 AIP patients. This study has raised to 18 the number of different mutations so far found among the Italian AIP population, 11 of which are new findings. We can conclude that the mutation screening in the Italian population contributes to improvement of the diagnostic approach of AIP and to establishing possible clustering of mutations in the Mediterranean area.