Human natural killer (NK) cell, which is an important lymphocyte for immune surveillance, is highly sensitive to heat, but the nature of its response to and its mechanistic regulation by heat remain unclear. Here we determined the effect of in vitro heat shock and in vivo hyperthermia on human NK cell cytotoxicity. Human peripheral blood mononuclear cells (PBMC) obtained from healthy volunteers were subjected to heat shock in vitro (42 degrees C, 1 h). PBMC from cancer patients receiving intentional hyperthermia (42 degrees C, 1 h) for cancer therapy were also obtained. NK cytolytic activity was determined in these samples. NK cell cytotoxicity was down-regulated by heat shock in vitro at 5 h, but at 24 h after heat shock, the NK cytotoxicity was comparable to that with its respective control. Furthermore, we observed that the mRNA and protein expression levels of perforin, which is the cytolytic granule of NK cells, were regulated by heat shock in a similar manner as NK cytotoxicity at 5 h and at 24 h after heat shock. Heat regulation involved the perforin protein in CD56(dim) but not in CD56(bright) NK cell subset. Heat shock neither induced cell death nor altered the expression of some NK activating receptors and adhesion molecules. Moreover, whole-body hyperthermia at 42 degrees C for 1 h of cancer patients also suppressed the cytotoxicity of NK cells but recovered to basal level 1 week after hyperthermia. Heat shock in vitro and in vivo temporarily represses the cytotoxicity of human NK cells.