Background: Although infected necrosis is an established cause of death in acute pancreatitis, the impact of bacteraemia and pneumonia is less certain. Methods: This was a cohort study of 731 patients with a primary episode of acute pancreatitis in 2004–2007, including 296 patients involved in a randomized controlled trial to investigate the value of probiotic treatment in severe pancreatitis. Time of onset of bacteraemia, pneumonia, infected pancreatic necrosis, persistent organ failure and death were recorded. Results: The initial infection in 173 patients was diagnosed a median of 8 (interquartile range 3–20) days after admission (infected necrosis, median day 26; bacteraemia/pneumonia, median day 7). Eighty per cent of 61 patients who died had an infection. In 154 patients with pancreatic parenchymal necrosis, bacteraemia was associated with increased risk of infected necrosis (65 versus 37·9 per cent; P = 0·002). In 98 patients with infected necrosis, bacteraemia was associated with higher mortality (40 versus 16 per cent; P = 0·014). In multivariable analysis, persistent organ failure (odds ratio (OR) 18·0), bacteraemia (OR 3·4) and age (OR 1·1) were associated with death. Conclusion: Infections occur early in acute pancreatitis, and have a significant impact on mortality, especially bacteraemia. Prophylactic strategies should focus on early intervention. Copyright © 2009 British Journal of Surgery Society Ltd. Published by John Wiley & Sons, Ltd. Infection occurs early