Post endoscopic retrograde cholangiopancreatography (ERCP) pancreatitis is a frequent complication either for diagnosis or treatment of pancreatobiliary diseases. A number of pharmacological agents have been tried for prevention or alleviation of the complication. Allopurinol with free radical scavenger property has been considered as an effective prophylactic agent in some clinical trials. Administration of allopurinol in these trials was done in a long period before doing ERCP. Hence allopurinol converts to oxupurinol in the liver rapidly; it seems that clinical judgment about the net effect of allopurinol on prevention of post ERCP pancreatitis is doubtful. In this randomized double blind clinical trial, effect of allopurinol on prevention or alleviation of clinical and laboratory signs of pancreatitis has been evaluated in 74 patients undergoing ERCP. Results showed that there is not any difference between allopurinol and placebo in occurrence and severity of post ERCP pancreatitis (P=0.97). Also there is not any significant difference in amylase rises between 2 groups in 8 and 16 hours after ERCP (P=0.947, 0.287 respectively). Beneficial effects of allopurinol in some of the previous studies may be attributed to its active metabolite (oxypurinol). Further studies recommended about the net effect of allopurinol and oxypurinol in the complication.