The effects of environmental temperature, presence and severity of El Niño, presence of cholera in the community, and interactions among these variables on the number of adult diarrhoeal patients attending the Hospital Nacional Cayetano Heredia in Lima, Peru, during 1991-1998, were evaluated. During 1991-1996, an increased number of visits to the hospital due to acute diarrhoea in the warmer months was observed. This periodic pattern was altered in 1997, when rising of the environmental temperature was observed in Lima secondarily associated with a strong El Niño event. A multivariate model was built in which environmental temperature and interaction between environmental temperature and presence of cholera predicted the number of adult patients with acute diarrhoea attending the Hospital Nacional Cayetano Heredia. Monitoring of environmental temperature and presence of cholera may be used as a warning system to predict epidemics of diarrhoea in adults, which may have a tremendous impact on healthcare strategies and management of health services in general. © 2004 ICDDR,B: Centre for Health and Population Research.