Electromagnetic radiation ranging from radiofrequency to microwave has classically been used to induce hyperthermia for treatment of cancer. This paper presents a new technique using near infrared radiation from an Nd:YAG laser in conjunction with surface cooling to induce hyperthermia in a rat tumor model. A CW Nd:YAG laser hyperthermia system was used to induce hyperthermic temperatures in chemically (DMBA) induced rat mammary adenocarcinomas. The laser was interfaced to a computer and a thermometry unit that provided feedback to control the tumor temperature between 43.2-43.5 degrees C. A thermocouple was placed at the base of the tumor and its temperature was used to control laser exposure. All tumors were 10 to 20 mm in diameter. Surface cooling methods investigated included forced air flow from a fan, forced oxygen flow plus an IV drip, and forced moist oxygen flow from a nebulizer. Twelve rat mammary adenocarcinomas have been treated with Nd:YAG laser hyperthermia. In 4 treatments, no surface cooling was employed. In one treatment the surface was cooled using oxygen flow plus IV drip. In 7 treatments the skin was cooled using the nebulizer technique. The nebulization provided the most effective and reproducible surface cooling. Nd:YAG laser hyperthermia delivered in conjunction with nebulizer surface cooling produced efficient heating of rat mammary adenocarcinomas. A mean temperature of 42.1 degrees C was obtained at the base of the tumors while the mean surface temperature was 37.0 degrees C.