A number of hereditary syndromes have been found to be associated with pheochromocytoma development, but there is a paucity of data regarding secondary molecular events, such as downregulation of the p16INK4A/Cdkn2a gene (hereafter p16), contributing to pheochromocytoma tumorigenesis. Using tissue microarray and immunohistochemistry, we evaluated the expression of p16 in 31 pheochromocytoma tumor specimens. Our results showed that the p16 gene was expressed at low level or even not expressed in all but one specimens [30/31 (96.8%)], indicative of the prevalence of p16 downregulation in pheochromocytomas. In contrast, high expression of pl6 was observed in the majority of control "normal" specimens [5/7 (71.6%)]. To further investigate the molecular mechanisms underlying pl6 downregulation in pheochromocytomas, we used quantitative real-time PCR, methylation-specific PCR, and direct DNA sequencing to analyze these specimens for potential genetic alterations of the p16 gene. Deletions and aberrant CpG methylation of pl6 were identified in 9 (29.0%) and 11 (35.5%) specimens, respectively, while one specimen harbored a point mutation, Ala --> Pro at residue 20 of P16, and this mutation led to an eightfold decrease in the CDK4-inhibitory activity of P16. The overall frequency of pl6 genetic alterations is 67.7%. Taken together, our results demonstrate that reduced expression of pl6 is a common event in human pheochromocytomas, and the primary cause for such downregulation is inactivating genetic abnormalities in the p16 gene.