Unusually high temperatures believed to represent the El Nino phenomenon have been accompanied by a marked increase in diarrhea and dehydration cases in Lima Peru. To verify this relationship monthly statistics on diarrhea and dehydration patients admitted to a rehydration unit in Lima during 1993-97 were matched with data on temperature obtained from 3 weather stations of the National Service for Meteorology and Hydrology in northern Lima. The 1997 data represent an abrupt deviation from previously recorded patterns for both diarrhea and temperature. In 1997 the temperature remained above 20 C through September rather than dropping below this level in May as occurred in the previous 4 years. During April-June 1997 the number of diarrhea cases increased by 35% relative to the first quarter in contrast to the 16% average decrease recorded in previous years. Although diarrhea cases began to fall off in the third quarter of 1997 the total number was 25% above figures from earlier years. These findings suggest grounds for concern that the high seasonal temperatures in the summer of 1998 exacerbated by El Nino will be associated with an outbreak of diarrhea if not a cholera epidemic.