Recently, we have cloned and characterized the Ras association domain family 1A gene (RASSF1A) at 3p21.3, from which loss of genetic material is one of the most frequent events in several types of human solid tumors. The CpG island promoter region of this gene is highly methylated in several human cancers, most notably in small cell lung cancer, breast cancer, and renal cell carcinoma. In this study, we have analysed the methylation status of RASSF1A in primary prostate tumors and in the prostate cancer cell line LNCaP. In total, 37 out of 52 tumors (71%) were methylated at the promoter region of RASSF1A. The relative frequency of methylation was higher in more aggressive tumors compared with less malignant tumors. For instance, tumors with a Gleason score of 7-10 (25 out of 30, 83%) were significantly more methylated compared with Gleason 4-6 tumors (11 out of 20, 55%, P=0.032, Fisher's exact test). Coincident with a hypermethylated promoter, transcripts of RASSF1A were missing in LNCaP cells. Expression of RASSF1A was restored with 5-aza-2'-deoxycytidine, a DNA methylation inhibitor. In conclusion, our data suggest that epigenetic inactivation of RASSF1A by methylation is a very common event in prostate cancer and might be involved in the progression of the disease. Testing for RASSF1A methylation should become useful in prostate cancer early detection and diagnosis and might aid prognosis by gauging the potential status of progression.