We analyzed heating profiles from 318 hyperthermic treatments of 39 patients with recurrent or inoperable cancers of the digestive organs whose deep-seated tumors were treated by radiofrequency (RF) capacitive heating of the abdominal region, and we investigated the heating efficiency and antitumor effect of such treatment. It was apparent that heating with a mean maximum RF output of 1,000 watts (700 watts at least), repeated four times or more, was necessary for a high rate of response by the tumor. Although it was difficult to heat tumors of the bile duct/pancreas to 42 degrees C or more, there was a strong positive correlation between maximum output of RF energy and maximum temperature of tumors (r = 0.839, P less than 0.001). The antitumor effect of RF hyperthermia was augmented with increasing output of RF energy. Therefore, the maximum level of RF output may be a useful index for expressing the heating efficiency with respect to intra-abdominal deep-seated tumors.