The concept of critical thermal maximum (CTM) has been defined in the literature as the minimal high deep-body temperature that is lethal to an animal. In man the CTM has been estimated at 41.6--42.0 degrees C. Data are presented for sedated, unacclimatized, well-hydrated men heated 1 h at esophageal temperatures of 41.6--42.0 degrees C, without sequelae, except for modest elevation of serum enzymes in two of five patients. These data when combined with other observations in the literature suggest that CTM be redefined as the particular combination of exposure time at elevated body temperatures that results in either subclinical (CTM)s) or clinical (CTMc) injuries. Also presented is a mathematical technique, equivalent time at 42 degrees C (Teq 42 degrees), for expressing hyperthermia in terms of body temperature and exposure time.