Renal angiomyolipomas are benign tumors composed of variable amounts of mature fat, smooth muscle, and thick-walled blood vessels. They occur either sporadically or in association with tuberous sclerosis. Such tumors are considered as hamartomas, but few data are available concerning their pathogenesis. Indeed, it is not known whether angiomyolipoma is a congenital malformation or a neoplastic process. To answer this question, we assessed the clonality of sporadic angiomyolipomas using molecular analysis. Seven women (mean age, 59 years) with renal angiomyolipomas were included. DNA of the tumor and the normal adjacent kidney was extracted from archival paraffin-embedded tissue. DNA methylation pattern at a polymorphic site on the HUMARA gene was studied by polymerase chain reaction (PCR) amplification after methylation-sensitive enzyme digestion. This procedure enables the differentiation between polyclonal and monoclonal lesions according to their X-chromosome inactivation pattern. Five of the seven women included were informative for the HUMARA gene. The mean size of the angiomyolipomas was 53 mm (range, 18 to 110). In one case, a tumor thrombus was observed in the inferior vena cava. Clonal analysis showed that all of the angiomyolipomas and the tumor thrombus studied were monoclonal. This study shows that sporadic angiomyolipomas are monoclonal lesions consistent with neoplastic disorders. This result strongly supports the hypothesis that angiomyolipomas arise from the clonal proliferation of an uncommited cell, which will further evolve toward different cell types.