The blood concentrations of endotoxin in dogs with pyometra (n=45) were compared with those in healthy dogs (n=17). The blood endotoxin concentrations in the healthy dogs (n=17), in those with good prognosis (n=41) and those with poor prognosis (n=4) were 3.4 ± 2.8 pg/ml, 9.5 ± 11.3 pg/ml and 74.2 ± 18.3 pg/ml, respectively. The concentrations in the dogs with good prognosis and poor prognosis were significantly (p<0.01) higher than those in the healthy dogs. The dogs with poor prognosis had significantly (p<0.01) higher endotoxin concentrations than those with good prognosis. Blood endotoxin concentrations were measured in 9 dogs after surgery, and were found to be decreased. These results suggest the possible involvement of endotoxin in the pathophysiological changes due to pyometra in dogs, and also that the blood endotoxin concentration could be used as a marker to determine prognosis.