In 1984 we developed laser hyperthermia (hereafter called laserthermia), a system for transmitting low power Nd:YAG laser energy (2 W) through an interstitial probe for local hyperthermia (43-44°C). We have investigated this method in experimental and clinical studies, and reported some of our results at the last conference. However, since the process by which laser thermia induces tumor destruction is still not fully understood, we first performed in-vivo and, later, in-vitro and detailed in-vivo studies and obtained the following results: 1) There are differences in the process of carcinoma cell destruction between laserthermia(low power, long time) and contact irradiation (high power, short time). 2) Changes due to laserthermia become detectable histologically 3h after irradiation. 3) Laser-light itself has the ability to cause carcinoma cell destruction. Consequently, we suggest that laserthermia has a synergistic effect of hyperthermia (thermal effect) and laser-light. In further clinical studies, laserthermia has proved to be safer and more reliable than conventional non-contact or contact laser irradiation in eradicating tumor tissue.