Although photodynamic therapy (PDT) is becoming an additional cancer therapy, only little is known about its interactions with other drugs and treatment modalities in vitro and in vivo. We investigated the combination of 5-fluoro-2'-deoxyuridine (5FdUr), a chemotherapeutic drug, with 5,10,15,20-tetra( m-hydroxyphenyl)chlorin (mTHPC), a potent photosensitizer. Two cell lines, MCF-7 and LNCaP, were either simultaneously or consecutively incubated with both drugs and irradiated with laser light to activate mTHPC, and cell survival was determined. The combination of the two treatments was additive or antagonistic in LNCaP cells but additive or synergistic in MCF-7 cells depending on the protocol and concentration of 5FdUr. In one patient with multiple basal cell carcinoma, the effect of the combination of 5-fluorouracil administration followed by PDT resulted in significantly stronger effects than expected, leading to severe oedema, redness and ulceration. The healing process was delayed by 2 months compared to PDT alone. It is therefore important to find optimal conditions under which PDT and chemotherapy combinations do not fall outside the therapeutic window.