Quantitative studies were performed to determine how mild hyperthermia (HT, 37--42°C) modulates photodynamic therapy (PDT) using a sub-cellular targeting photosensitizer aluminum phthalocyanine tetrasuflonate (AlPcS 4) and a vascular-targeting photosensitizer Pd-bacteriopheophorbide (WST09). The ultimate goal is to improve WST09-mediated PDT treatments of prostate cancer, which are currently under investigation at the University Health Network. Simultaneous HT enhanced WST09-mediated PDT of rat prostate endothelial cells in vitro (HER of 1.42--1.67 at 41--42°C). Using the PDT threshold dose as the in vivo metric, the first quantitative relationship between temperature and PDT threshold dose was determined. HT (40--42°C) reduced the PDT threshold dose by 48 +/- 10% and 27 +/- 9% for WST09 and AlPcS4 respectively. Finally, by coupling the in vivo data with a mathematical model of optical/thermal propagation in tissues, it was shown that adjuvant HT may be an effective strategy to modulate PDT treatment volumes or homogeneity of response.