Acute pouchitis (AP) after ileal pouch-anal anastomosis (IPAA) is common and easily treated. However, chronic pouchitis (CP) remains a difficult management problem and may represent a form of Crohn disease (CD) of the ileal pouch. Because CD patients have higher platelet counts than ulcerative colotis (UC) patients, we prospectively evaluated the association between preoperative platelet count and pouchitis development in 159 patients undergoing IPAA. Reactive thrombocytosis (RT) was defined as a platelet count > 450 x 10(9)/L. Median preoperative platelet count was 312 x 10(9)/L (range, 103 x 10(9)/L to 886 x 10(9)/L). One hundred twenty-five patients (79%) had a normal (150 x 10(9)/L to 450 x 10(9)/L) platelet count (-RT patient group). Twenty-eight patients (18%) had RT. Six patients (3%) had a platelet count below 150 x 10(9)/L. After a median follow-up of 13 months, 45 patients (28%) developed pouchitis. Pouchitis developed in 33 +RT patients (26%) versus 9 -RT patients (32%) (P = NS). UC patients who had +RT had a 25 per cent incidence of CP compared to only 7 per cent of those UC patients who had -RT (P = 0.03). The incidence of CP was significantly higher after IPAA in UC patients having thrombocytosis before surgery compared to UC patients having a normal platelet count before surgery.