Growth-inhibition of GW-77 human colonic tumors growing in hamster cheek pouches was found to be proportional to the duration of heating the exposed tumors by 4 °C with short-wave diathermy. A maximal tumor growth-inhibition of 35 per cent resulted from administration of a daily 30 min. heating period repeated on 7 consecutive days. Further, hyperthermia to the left cheek pouch tumors resulted in growth-inhibition of the contralateral, presumably normothermic, cheek pouch tumors, thus causing us to question whether hyperthermia has abscopal antitumor effects.