To identify the frequency and associated factors of persistent diarrhoea in a paediatric hospital in Tehran, Islamic Republic of Iran, children admitted with acute diarrhoea were followed prospectively until resolution of the episode. Persistent diarrhoea developed in 19.6% of the 424 diarrhoea cases. Children with persistent diarrhoea were significantly younger than those with acute diarrhoea (15.3 versus 29.4 months). Enteroaggregative Escherichia coli was the most prevalent pathogen in both groups, but otherwise the profile of symptoms and isolated organisms was similar in the 2 groups. History of dietary change prior to admission and use of antibiotics and anticholinergic drugs in the acute phase were significantly higher in the persistent than acute diarrhoea cases.