Aberrant promoter methylation and resultant silencing of several tumor suppressor genes play an important role in the pathogenesis of many tumor types. The human Ras association domain family 1A gene (RASSF1A), recently cloned from the lung tumor locus at 3p21.3, was shown to be frequently inactivated by hypermethylation of its promoter region in a number of malignancies. We have investigated the expression and epigenetic changes of this novel universal tumor suppressor gene in pituitary adenomas and correlated the data with clinicopathologic findings. Fresh frozen normal pituitary tissues and 52 primary pituitary adenomas including all major types were examined. Methylation-specific polymerase chain reaction (MSP), combined bisulfite restriction analysis (COBRA), bisulfite sequencing and semiquantitative reverse transcription-polymerase chain reaction were used to analyze DNA promoter methylation status and the mRNA expression of RASSF1A, respectively. High levels of RASSF1A transcript and no methylation of the RASSF1A promoter were found in normal pituitary tissues. RASSF1A promoter methylation was detected in 20 of 52 (38%) adenomas including all major types of pituitary adenomas. However, a lower frequency of methylation of the RASSF1A promoter was found in gonadotroph cell adenomas (15%) compared with growth hormone cell, prolactin cell, or adrenocorticotropic hormone cell adenomas (54, 46 and 50%, respectively). Methylation frequency was higher in the most aggressive adenomas (87% in grade IV tumors, P=0.0163). In addition, methylation of the RASSF1A promoter potentially correlated with higher labeling index of the proliferation marker Ki-67 (P=0.1475). Loss or significant reduction of RASSF1A messenger RNA transcripts was identified in 18 of 20 (90%) adenomas with hypermethylation of RASSF1A (P<0.0001). Our data suggest that promoter hypermethylation of RASSF1A and resultant alterations of RASSF1A expression may play a critical role in pituitary tumorigenesis and may be involved in tumor progression.