In photodynamic diagnosis, 5-aminolevulinic acid (5-ALA) is widely used for the fluorescence-guided resection of malignant brain tumors, where 5-ALA is converted to protoporphyrin IX, which exhibits strong fluorescence. Little is known, however, about the detailed molecular mechanisms underlying 5-ALA-induced fluorescence. To resolve this issue, we analyzed transcriptome profiles for the genes encoding enzymes, transporters, and a transcription factor involved in the porphyrin-biosynthesis pathway. By quantitative real-time (qRT)-PCR, we measured the mRNA levels of those genes in a total of 20 tumor samples that had been surgically resected from brain tumor patients at the Department of Neurosurgery of Osaka Medical College from 2008 to 2009. We selected 10 tumor samples with no 5-ALA-induced fluorescence, among which 2 were glioblastomas and 8 were metastatic brain tumors. Another 10 tumor samples were selected with strong fluorescence, among which 7 were glioblastomas and 3 were metastatic brain tumors. The qRT-PCR analysis study of these latter 10 samples revealed predominantly high levels of the mRNA of the coproporphyrinogen oxidase (CPOX) gene. The high mRNA level of CPOX expression was significantly well correlated with the phenotype of strong 5-ALA-induced fluorescence (P = .0003). These findings were further confirmed by immunohistochemical studies with a CPOX-specific antibody. It is concluded that induction of CPOX gene expression is one of the key molecular mechanisms underlying the 5-ALA-induced fluorescence of malignant brain tumors. The induction mechanism for the CPOX gene in brain tumors remains to be elucidated.