Previous findings have suggested that methylation of the APC gene may be associated with some tumors including lung cancer. To explore the pattern of APC methylation and the effect of APC gene methylation on its protein expression in lung cancer cell lines, we investigated APC promotor methylation by methylation specific PCR (MSP) and bisulfite sequencing and analyzed the APC protein levels by western blot in three lung cancer cell lines. Monoallelic methylation and 20 methylated CpGs in CpG island near the open reading frame (ORF) of the APC gene were found in the NCI-H460 cell line, and were stablely inherited within 10 generations of the cell line in culture. Our results showed that two special CpG sites (794, 797) might be binding sites for proteins that regulate APC expression. Protein expression of the APC gene in the NCI-H460 cell line declined, but was enhanced after the treatment with 5-aza-2-deoxycytidine (5-aza-dC). Inherited monoallelic methylation of the APC gene may play an important role in lung cancer. Demethylation of the APC gene by 5-aza-dC may be useful for the treatment of lung cancer.