Photodynamic therapy (PDT) and hyperthermia have been investigated as treatments for several types of tumors. Studies have been done to determine the efficacy of each modality individually and recently in combination with each other. In this study, 630-nm light was delivered by an argon-dye laser and hyperthermia was induced using an Nd:YAG laser. Both lasers offer the ability of delivering the beams through a quartz fiberoptic alone or simultaneously. This study examines the efficacy of the simultaneous administration of PDT and selective hyperthermia at 44.5 degrees C in tumor control in the spontaneous mammary tumor (SMT-F) in DBA mice. Hyperthermia alone (44.5 degrees C, 30 min) resulted in complete destruction of tumors, with no subsequent regrowth in 6.6% of the mice treated. PDT alone (5 mg/kg dihematoporphyrin ether; 135 J/cm) resulted in a cure rate of approximately 10%, and the simultaneous treatment of the modalities resulted in a 32.8% cure rate after 90 days. These values are indicative of a synergistic interaction.