OBJECTIVE A genetic variation in the androgen receptor (AR) has been associated with the risk of developing endometriosis. The AR gene is located on the X chromosome and contains a highly polymorphic trinucleotide repeat (cytosine, adenine, and guanine: CAG) in its first exon, whose length and methylation pattern affect both AR expression and function. Thus, we sought to further investigate the potential association between endometriosis and the AR-CAG polymorphism. METHODS Genomic DNA was obtained from a consecutive series of 197 white Italian women of reproductive age who underwent laparoscopy for benign gynecologic pathologies. Molecular analysis of AR-CAG repeats was performed by polymerase chain reaction amplification and Genescan evaluation. The pattern of CAG repeat distribution was compared between subjects with and without endometriosis. RESULTS Endometriosis was documented in 105 women (stage I-II in 33 women and stage III-IV in 72 women). We found no difference in the number of AR-CAG repeats between women with endometriosis and controls. The CAG repeat length ranged from eight to 27 (mean +/- standard deviation, 17.4 +/- 1.9) for endometriosis patients and from 11 to 27 (mean +/- standard deviation, 17.4 +/- 2) for controls. Moreover, no association was found between AR gene polymorphisms and the various clinical manifestations of the disease. CONCLUSION We conclude that AR-CAG repeat length does not constitute an important factor for the genetic predisposition to endometriosis.