Objective  To investigate the role of methylated DNA mismatch repair gene MLH1 and MSH2 in the acquired multidrugresistance of human small cell lung cancer cells H446.   Methods  The reverse transcription polymerase chain reaction (RT-PCR) and Western blot were applied to measure MLH1 and MSH2 mRNA and protein expressions of the multidrug-resistant cells H446/DDP and its parental cells H446. The promoter methylation status of the genes was assessed by  methylation-specific PCR (MSP).   Results  The expressions of MLH1 and MSH2 significantly decreased both in mRNA level and protein level. Promoter methylation of MLH1 was observed in H446/DDP cells but not in H446 cells. Promoter semi-methylation of MSH2 in H446 cells was transformed to methylation in H446/DDP cells.   Conclusion  The downregulation of DNA mismatch repair gene MLH1 and MSH2 induced by its promoter methylation may play an important role in the acquired multidrug resistance of human small cell lung cancer.