Acute intermittent porphyria (AIP) is an autosomal dominant disorder characterized by alterations of the gene encoding porphobilinogen deaminase (PBGD: EC 4.3.1.8), the third enzyme of the heme biosynthetic pathway. The molecular heterogeneity of the mutations causing AlP has been demonstrated with a reported predominance of single base substitutions resulting in amino acid changes. The molecular basis of AIP in four French patients was investigated using denaturing gradient gel electrophoresis followed by direct sequencing. We describe four different novel mutations that affected exon 12 (a frameshift and an exon skipping), exon 4 (a stop codon) and exon 15 (a frameshift inducing a stop codon). This study further documents the molecular heterogeneity of mutations in the PBGD gene in the French Caucasian population and reports types of mutations relatively uncommon in AIP.