Diarrhoea is common in Intensive Care Unit (ICU) patients, with a reported prevalence of 15-38%. Many factors may cause diarrhoea, including Clostridium difficile, drugs (e.g. laxatives, antibiotics) and enteral feeds. Diarrhoea impacts on patient dignity, increases nursing workload and healthcare costs, and exacerbates morbidity through dermal injury, impaired enteral uptake and subsequent fluid imbalance. We analysed a cohort of 9331 consecutive patients admitted to a mixed general intensive care unit to establish the prevalence of diarrhoea in intensive care unit patients, and its relationship with infective aetiology and clinical outcomes. We provide evidence that diarrhoea is common (12.9% (1207/9331) prevalence) in critically ill patients, independently associated with increased intensive care unit length of stay (mean (standard error) 14.8 (0.26) vs 3.2 (0.09) days, p < 0.001) and mortality (22.0% (265/1207) vs 8.7% (705/8124), p < 0.001; adjusted hazard ratio 1.99 (95% CI 1.70-2.32), p < 0.001) compared to patients without diarrhoea even after adjusting for potential confounding factors, and infrequently caused by infective aetiology (112/1207 (9.2%)) such as Clostridium difficile (97/1048 (9.3%) tested) or virological causes (9/172 (5.7%) tested). Our findings suggest non-infective causes of diarrhoea in ICU predominate and pathophysiology of diarrhoea in critically ill patients warrants further investigation.