OBJECTIVE Evidence supporting the value of preoperative antibiotic prophylaxis (PAP) in surgery for acute cholecystitis is lacking. This study aimed to shed light on whether PAP in acute cholecystectomy for cholecystitis reduces the postoperative infectious complication (PIC) rate. Secondary outcomes were the prevalence of bacteriobilia, CRP values and leucocyte counts. METHODS The study was performed as a single-centre, double-blinded, placebo-controlled, randomised study. Patients with acute cholecystitis amenable for acute laparoscopic cholecystectomy were randomly assigned to either PAP (piperacillin/ tazobactam) or placebo, and the subsequent clinical course was followed. RESULTS A total of 106 patients were enrolled, 16 of whom were excluded due to protocol violation. PIC developed in 22 of the 90 patients included with no significant difference between the PAP and placebo groups (8 patients in the PAP group and 14 in the placebo arm, p = 0.193). The PIC rate was significantly higher in patients with a raised CRP at randomisation and on the day of surgery and in cases of conversion to an open procedure (p = 0.008, 0.004 and 0.017, respectively) but with no differences between the study groups. CONCLUSIONS PAP does not affect the risk for PIC in patients with acute cholecystitis. The major risk factors determining PIC in these patients need defining, in particular, the impact of bacteriobilia. BACKGROUND The study was registered at clinicaltrials.gov (NCT02619149) December 2, 2015.