Patients with inflammatory bowel disease are susceptible to thromboembolism and recently small vessel thrombosis has been implicated as an aetiological factor in Crohn's disease. This study therefore investigated platelet function in 104 patients with inflammatory bowel disease of whom eight had previous thromboembolism. Thirty five patients had reproducible spontaneous platelet aggregation of more than 30% (0 in controls) (p < 0.0001). A further 20 patients showed hypersensitivity of platelets to low concentrations of aggregating agents (p < 0.001). Plasma thromboxane B2 and beta thromboglobulin levels were significantly higher than controls (p < 0.001 and p < 0.001), but platelet lifespan studies were normal. There was no correlation with disease activity. Patients with inflammatory bowel disease have abnormal platelet activity, which may contribute to the inflammatory process.