Thermotherapy is supposed to induce irreversible cell damage in tumour cells. Aim of this study was to evaluate the antiproliferative activity of temperatures between 45 and 65°C on various cancer cell lines and to compare the biological effects of water-bath and microwave induced thermotherapy, on cell proliferation. Cell cultures from five different tumours (DU-145, PC3, RTEJ, K562, Daudi) were placed in a precision water-bath and maintained for 1 hour at temperatures ranging between 37 and 65°C (experiment 1). Cell cultures from DU-145 and PC3 tumours were exposed for 1 hour at 50.4°C either in a precision water-bath or in a microwave field (>1 GHz frequency) (experiment 2). The antiproliferative effect of thermotherapy was evaluated, in both experiments, by 3H-thymidine uptake. Experiment 1: water-bath induced thermotherapy caused significant inhibition of cell proliferation with great variability among the different cell lines: 15.5-84.1% inhibition at 45°C, 19.1-89.4% at 50°C, 50.2-93.9% at 55°C, 77.5-97.5% at 60°C, 70.7-99.6% at 65°C. Experiment 2: one-hour thermotherapy treatment at 50.4°C induced 54.1% (microwaves) and 41.3% (water-bath) inhibition rates in PC3 cell line; 81.1% (microwaves) and 76.5% (water-bath) inhibition rates were observed in DU-145 cell line. In conclusion, a significant, although small, percentage of tumour cells survived 1 hour exposure to temperatures between 45 and 65°C. Antiproliferative effect of microwave thermotherapy seems to be related to thermal effects of the electromagnetic radiation.