OBJECTIVE: Acute pancreatitis is the most dreaded complication of ERCP. Two studies have shown a significant effect of glyceryl nitrate (GN) in preventing post-ERCP pancreatitis (PEP). We wanted to evaluate this promising effect in a larger study with a realistically precalculated incidence of PEP. DESIGN/PATIENTS: A randomized, double-blind, placebo-controlled multicenter study including patients from 14 European centers was performed. A total of 820 patients were entered; 806 were randomized. INTERVENTION: The active drug was transdermal GN (Discotrine/Minitran, 3M Pharma) 15 mg/24 hours; placebo (PL) was an identical-looking patch applied before ERCP. A total of 401 patients received GN; 405 received PL. RESULTS: Forty-seven patients had PEP (5.8%), 18 (4.5%) in the GN group and 29 (7.1%) in the PL group. The relative risk reduction of PEP in the GN group of 36% (95% CI, 11%-65%) compared with the PL group was not statistically significant (P = .11). Thirteen had mild pancreatitis (4 in the GN group, 9 in the PL group), 26 had moderate pancreatitis (9 in the GN group, 17 in the PL group), and 8 had severe pancreatitis (5 in the GN group, 3 in the PL group). Headache (P < .001) and hypotension (P = .006) were more common in the GN group. Significant variables predictive of PEP were not having biliary stones extracted; hypotension after ERCP; morphine, propofol, glucagon, and general anesthesia during the procedure; or no sufentanil during the procedure. CONCLUSIONS: The trial showed no statistically significant preventive effect of GN on PEP. Because of a considerable risk of a type II error, an effect of GN may have been overlooked. (ClinicalTrials.gov ID: NCT00121901.).