The purpose of this study was to evaluate the diagnostic and prognostic significance of tumor necrosis factor-alpha (TNF) and interleukin-6 (IL-6) activities and endotoxin concentration in blood and peritoneal fluid of 155 adult horses with acute abdominal disease (colic). Samples also were obtained from 20 healthy adult horses. Blood and peritoneal fluid supernatant TNF and IL-6 activities and endotoxin concentration were significantly greater in horses with colic, compared with healthy horses. In horses with colic, the peritoneal fluid endotoxin concentration and TNF and IL-6 activities were significantly greater than those in blood. Within the colic group, peritoneal fluid IL-6 activity was the analyte that was most frequently increased. Blood and peritoneal fluid supernatant TNF and IL-6 activities were significantly greater when endotoxin was detected in the same sample. Blood and peritoneal fluid IL-6 activity was significantly greater in horses with inflammatory or strangulating lesions, compared with horses having nonstrangulating or noninflammatory lesions. Compared with all other data categories, diagnostic accuracy for nonsurvival was greatest (80%) when blood IL-6 activity exceeded 60 units/mL. The results of this study indicate that endotoxin was present in the peritoneal cavity of at least one third of horses with any acute disease of the abdomen. In horses presented for colic, blood or peritoneal fluid IL-6 activity was more useful than either TNF activity or endotoxin concentration for distinguishing lesion type. Although diagnostic accuracy for the prediction of nonsurvival was good for all of the analytes, negative values were more useful in the prediction of a favorable outcome than were abnormally increased values in the prediction of mortality.