The aim of this study was to investigate the mechanism of cell death by photodynamic therapy (PDT) in the gastric cancer cell line MKN45 with focus on the mechanism of apoptosis. Gastric cancer cells (MKN45) were incubated with Photofrin for up to 24 h before exposure to He-Cd laser (441 nm, 1 J/cm2). Cell viability was assessed by the methyl-tetra-zolium assay after exposure to light. A 95% cell death (LD95) was measured with 10 microg/ml of Photofrin. DNA ladder formation and chromatin condensation were seen within 60 min. Caspase-3-like and caspase-9-like activities increased from 15 min after exposure to light. Reduction of rhodamine 123 uptake started at 30 min. Caspase-inhibitor VAD-fmk (10 mM) inhibited apoptosis, but did not influence cell viability. In conclusion, Photofrin-mediated PDT in the gastric cancer cell line MKN45 induces apoptosis within 60 min, and mitochondrial damage is likely as the first event of apoptosis.