The purpose of this study was to examine the effects of methemoglobinemia caused by fire gases on blood cyanide concentrations in fire victims. Twenty-two fire victims with postmortem intervals of 8-48 h were involved. Blood cyanide concentrations at the time of death (C(0)) were estimated using the formula: C(0)=Ce(0.046 t) (C=blood cyanide concentration detected at autopsy, 0.046=first-order rate constant of cyanide disappearing from blood in corpses, and t=postmortem interval). Total (free and combined with cyanide) methemoglobin (MetHb) content was used to estimate the maximum capacity of MetHb for combining cyanide. Blood cyanide concentrations at the time of death were very high (5.32-6.47 mg/l) in five victims. Three showed high saturation (54.7-63.0%) of carboxyhemoglobin (CO-Hb) and elevated total MetHb contents (2.6-5.0%). MetHb at these levels is capable of scavenging up to 8.6-11.4 mg/l of blood cyanide. Thus, blood cyanide might have been completely combined with MetHb at the time of their death. In the remaining two victims, CO-Hb saturation was not high (30.9 and 37.9%) and no free MetHb was detected. As a result they may have exhibited severe toxic effects of cyanide at the time of their death. Our results indicate that MetHb contents and CO-Hb saturation should be determined to evaluate the toxic effects of cyanide in fire victims.