A total of 212 adult patients with infective diarrhoea and 27 with inflammatory bowel disease (IBD), admitted consecutively to an infectious disease unit, were studied in order to determine whether clinical features and laboratory measurements performed on admission identified cases of IBD. Long-standing diarrhoea, blood in the faeces, anaemia, leucocytosis, thrombocytosis, raised ESR and a reduced concentration of serum albumin were more common in patients with IBD (P less than 0.05). The most striking difference was in the platelet count with 59% patients with IBD and 1.6% patients with infective diarrhoea having platelet counts greater than 450 x 10(9)/l. A raised platelet count in a patient admitted to hospital with 'acute gastro-enteritis' suggests IBD.