BACKGROUND: 5-Aminolevulinic acid photodynamic diagnosis (ALA-PDD) is a method for real-time diagnosis of cancer areas based on the specific accumulation of protoporphyrin IX (PpIX) in tumor cells following ALA administration and subsequent detection using fluorescence. However, its effectiveness is limited due to false negatives caused by tumor cells accumulating low amounts of PpIX after ALA administration. In order to increase the detection sensitivity of ALA-PDD, it is important to understand the characteristics of cells with low or high PpIX accumulation. This research sought to determine the factors affecting ALA-induced PpIX accumulation in tumor cells METHODS: Tumor cells were separated into two groups according to PpIX accumulation levels using FACS and the cellular characteristics were compared. RESULTS: The data of expression of RANKL, a tumor malignancy marker, showed that tumor cells with low PpIX accumulation are more malignant. Results also showed marked differences in PpIX efflux transporter activity and the extent to which iron atoms are being inserted into the porphyrin ring, resulting in loss of fluorescence. Interestingly, it was found tumor cells with high PpIX accumulation are in dormant state as indicated by low proliferation and suppression of glucose metabolism. CONCLUSION: Tumor cells with low PpIX accumulation are characterized as high tumor grade and low cell dormancy.