[en] Background: Mutations in the Aryl hydrocarbon receptor Interacting Protein (AIP) gene have been described in about 15% of kindreds with Familial Isolated Pituitary Adenomas (FIPA) and in a minority of early onset sporadic pituitary adenomas (PA). Among the AIP mutations reported so far, the R304X (AIPR304X) represents, together with the "Finnish mutation" Q14X, the most common one. Methods: Three AIPR304X Italian families, including a newly reported kindred, have been genotyped for 12 genetic markers surrounding the AIP gene in order to look for a potential founder effect in Italy. Disease penetrance and genotype-phenotype correlations were also addressed. Results: Analysis of chromosome 11' genetic markers revealed a common haplotype in two AIPR304X kindreds originating from central Italy. Overall, seventeen mutations carriers were identified, including 7 patients and 10 unaffected subjects, respectively, arguing in this case for a disease penetrance of 41%. Mean age at diagnosis was 19.1+/-6.7 years-old, with females tending to be older than males. Though most PA were somatotropinomas (6/7), a great variability in disease severity was observed, even between subjects sharing the same at-risk haplotype. Conclusion: These data provide strong evidence for a new founder effect of the AIPR304X mutation in central Italy and the observed variations in disease severity point out the role of additional genetic or environmental factors in such kindreds.