The occurrence and distribution of Vibrio cholerae in sea water and plankton along the coast of Peru were studied from October 1997 to June 2000, and included the 1997–98 El Niño event. Samples were collected at four sites in coastal waters off Peru at monthly intervals. Of 178 samples collected and tested, V. cholerae O1 was cultured from 10 (5.6%) samples, and V. cholerae O1 was detected by direct fluorescent antibody assay in 26 out of 159 samples tested (16.4%). Based on the number of cholera cases reported in Peru from 1997 to 2000, a significant correlation was observed between cholera incidence and elevated sea surface temperature (SST) along the coast of Peru (P< 0.001). From the results of this study, coastal sea water and zooplankton are concluded to be a reservoir for V. cholerae in Peru. The climate–cholera relationship observed for the 1997–98 El Niño year suggests that an early warning system for cholera risk can be established for Peru and neighbouring Latin American countries.