Expression of the cell surface receptor Fas is frequently lost or decreased during tumor progression in human colon carcinomas. The methylation status of a 583 bp CpG-rich region within the Fas promoter (-575 to +8) containing 28 CpG sites was determined in human colon carcinoma cell lines. In Caco(2) (no Fas expression), 82-93% of CpG sites were methylated, whereas none were methylated in GC(3)/c1 (high Fas expression). In RKO (intermediate level of Fas), a single CpG site, located at -548, was 100% methylated. The inhibitor of methylation, 5-aza-2'-deoxycytidine (5-azadC), upregulated Fas expression in four of eight cell lines, and sensitized RKO cells to recombinant FasL-induced apoptosis. The p53-binding region in the first intron of the Fas gene was partially methylated in Caco(2), and 5-azadC potentiated Ad-wtp53-induced upregulation of Fas expression. Methylation-specific PCR of the first intron detected partial methylation in four out of 10 colon carcinoma tumor samples in vivo. The data suggest that DNA hypermethylation is one mechanism that contributes to the downregulation of Fas expression and subsequent loss of sensitivity to Fas-induced apoptosis in colon carcinoma cells.