When a patient presents with acute diarrhoea, the most important question is whether it represents the first episode of chronic inflammatory bowel disease (CIBD) or acute self-limited colitis (ASLC). The early changes in the rectal mucosa of patients with ASLC have been claimed to be highly diagnostic, the presence of an unspecific inflammatory infiltrate in the lamina propria and the absence of disturbances in the crypt architecture being the characteristic features. Thirty-two patients who in the period 1980-1984 presented with their first episode of acute diarrhoea and whose rectal mucosa showed these histologic features were followed up for 1.5-7 years. Forty-one per cent of the patients were finally classified as having ASLC and 28% as having diseases other than CIBD or ASLC. Thirty-one per cent eventually developed CIBD. Thus the features claimed to be diagnostic of ASLC could not predict the final outcome of the disease in patients presenting for the first time with acute diarrhoea. The histologic interpretation can provide significant information in these patients but should never replace a careful clinical judgement and, if doubt remains, further observation of the patients.