BACKGROUND The use of prophylactic antibiotics in acute severe necrotizing pancreatitis is controversial. METHODS Prospective, randomized, placebo-controlled, double-blind study was carried out at Bellvitge Hospital, in Barcelona, Spain. Among 229 diagnosed with severe acute pancreatitis, 80 had evidence of necrotizing pancreatitis (34/80 patients were excluded of the protocol). Forty-six patients without previous antibiotic treatment with pancreatic necrosis in a contrast-enhanced CT scan were randomly assigned to receive either intravenous ciprofloxacin or placebo. Five patients were secondarily excluded, and the remaining 41 patients were finally included in the study (22 patients received intravenous ciprofloxacin and 19 patients placebo). RESULTS Comparing the 22 with intravenous ciprofloxacin and 19 with placebo, infected pancreatic necrosis was detected in 36% and 42% respectively (p = 0.7). The mortality rate was 18% and 11%, respectively (p = 0.6). No significant differences between both treatment groups were observed with respect to variables such as: non-pancreatic infections, surgical treatment, timing and the re-operation rate, organ failure, length of hospital and ICU stays. CONCLUSIONS The prophylactic use of ciprofloxacin in patients with severe necrotizing pancreatitis did not significantly reduce the risk of developing pancreatic infection or decrease the mortality rate. The small number of patients included in this study should be considered.