One promising method to visualize cancer cells is based on the detection of the fluorescent photosensitizer protoporphyrin IX (PpIX) synthesized from 5-aminolevulinic acid (ALA), but this method cannot be used in cancers that exhibit poor PpIX accumulation. PpIX appears to be pumped out of cancer cells by the ABC transporter G2 (ABCG2), which is associated with multidrug resistance. Genistein is a phytoestrogen that appears to competitively inhibit ABCG2 activity. Therefore, we investigated whether genistein can promote PpIX accumulation in human lung carcinoma cells. Here we report that treatment of A549 lung carcinoma cells with genistein or a specific ABCG2 inhibitor promoted ALA-mediated accumulation of PpIX by approximately 2-fold. ABCG2 depletion and overexpression studies further revealed that genistein promoted PpIX accumulation via functional repression of ABCG2. After an extended period of genistein treatment, a significant increase in PpIX accumulation was observed in A549 cells (3.7-fold) and in other cell lines. Systemic preconditioning with genistein in a mouse xenograft model of lung carcinoma resulted in a 1.8-fold increase in accumulated PpIX. Long-term genistein treatment stimulated the expression of genes encoding enzymes involved in PpIX synthesis, such as porphobilinogen deaminase, uroporphyrinogen decarboxylase, and protoporphyrinogen oxidase. Accordingly, the rate of PpIX synthesis was also accelerated by genistein pretreatment. Thus, our results suggest that genistein treatment effectively enhances ALA-induced PpIX accumulation by preventing the ABCG2-mediated efflux of PpIX from lung cancer cells and may represent a promising strategy to improve ALA-based diagnostic approaches in a broader set of malignancies.