Abstract: Objective To investigate the effect of 5-aminolevulinic acid(5-ALA)-mediated photodynamic therapy(PDT) on MGC-803 human gastric cancer cells in vitro.Methods MGC-803 human gastric cancer cells were treated with 5-ALA at various concentrations followed by laser irradiation.The cells were also treated with 5-ALA at the same concentration before laser exposure at various doses.PDT-induced phototoxicity of the cells was determined by MTT assay.Results After laser exposure of the cells at the same dose(25.0 J/cm2),the cell survival rates decreased significantly with incubation of the cells with 5-ALA at 0.25,0.5,1.0,2.0 and 4.0 mmol/L,respectively(F=266.39,P<0.001),but 2.0 and 4.0 mmol/L ALA showed no significant difference in lowering the cell survival rates(P>0.05).Following treatment with the same 5-ALA concentration(1 mmol/L),the cell survival rates decreased in response to increased laser doses(at 6.25,12.5,25.0,50.0,and 100 J/cm2,respectively,F=226.31,P<0.0001).Without laser exposure,the survival rate of the cells did not significantly change for different 5-ALA concentrations(F=0.79,P=0.5383),nor did it undergo obvious variation in response to different laser doses without 5-ALA incubation(F=0.61,P=0.6551).Conclusions The damage of MGC-803 cells by PDT increases with 5-ALA concentration within a relative lower range and is proportional to the laser doses delivered.Without 5-ALA treatment,the laser at the chosen dose cannot produce photodynamic effect and ALA itself is nontoxic.ALA-mediated PDT appears to be a promising therapy for gastric cancer.