Acute mesenteric ischemia is extremely difficult to diagnose because peritoneal signs are absent until the bowel becomes necrotic and irretrievably damaged. So far the only reliable diagnostic procedure has been mesenteric angiography. The present study verifies that significant elevations of serum inorganic phosphate concentrations occur in dogs after mesenteric occlusion; the authors' initial clinical studies in humans support these experimental findings. Detailed analysis of body fluid and soft tissue phosphate content shows that the high phosphate load originates in the sloughing intestinal mucosa; if this phosphate escapes filtration by the liver it enters the systemic circulation. Simple measurement of the inorganic phosphate concentrations of the serum and peritoneal fluid may lead to earlier diagnosis of acute intestinal ischemia and a successful revascularization procedure to prevent its progression to infarction.