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The purpose of this paper is to assess the evidence for and against the premise that hyperthermia is carcinogenic. The paper is one of several published in this issue of the International Journal of Hyperthermia on the subject of the health risks of hyperthermia. The motivation for this issue of the journal was the result of a World Health Organization workshop that dealt with this issue, as it relates to exposure of the population to RF fields. Since hyperthermia can be a natural consequence of such exposures, the health risks of hyperthermia are relevant in this context. Particularly in the case of carcinogenesis, it is necessary to provide a brief overview of the data that have been generated to examine the carcinogenic risks of RF exposure, so that these results can be compared with studies that have examined the carcinogenic risks of hyperthermia. For this reason, the paper is organized into three sections dealing with: (1) effects of heat on DNA damage/repair and mutations, (2) in vivo studies evaluating the carcinogenic potential of heat alone and combined with other carcinogens, and (3) in vivo studies involving RF exposures. The bulk of the data presented indicate that hyperthermia alone is not carcinogenic. If hyperthermia occurs in the presence of exposure to known carcinogens, such as radiation or chemical carcinogens there is the potential for modulation of carcinogenic effects of those agents. In some circumstances, hyperthermia can actually protect against tumour formation. In other instances, hyperthermia clearly increases incidence of tumour formation, but this occurs following thermal exposures (several °C temperature rise for up to 1 h or more) and radiation (therapeutic levels as for treatment of cancer) or chemical carcinogen doses higher than would be encountered by the general population. The extrapolation of these results to the general population, where radiation exposure levels would be at background and temperature rise from... [ABSTRACT FROM AUTHOR]