A significant proportion of the illness and death of diarrhoeal diseases in the developing world is estimated to be due to the diarrhoea associated with measles. During February 1983-January 1984 a prospective study of measles in a hospital in Northeastern Thailand was conducted. A total of 550 cases of measles were studied. Diarrhoea was the most frequent complication of measles, occurring in 233 cases (42.4%). The largest proportion (46.2%) of cases with diarrhoea occurred in May-July. Children with measles aged 6-11 months had the highest frequency of diarrhoea (65.7%). Cases aged 1 year and 0-5 months had diarrhoea rates of 60% and 57% respectively. The proportion of measles cases with diarrhoea decreased with increasing age. Only 9.1% (9/99) of stools sent for bacteriological culture were positive. In three of these Shigella spp. were isolated. The rest were non-typhoid Salmonella (2), enteropathogenic Escherichia coli (2), Vibrio cholera (1), and Vibrio parahaemolyticus (1). Other complications among measles cases were pneumonia in 168 (30.5%), otitis media in 28 (5.1%), convulsion in 13 (2.4%), croup in 9 (1.6%), encephalitis in 4 (0.7%), and sepsis in 1 (0.2%). Seven cases (1.3%) died, 4 from pneumonia, 2 from encephalitis, and 1 from sepsis.