ABSTRACT Ciguatera (fish poisoning) is the most frequent cause of human illness caused by ingestion of marine toxins. The toxins are ingested by herbivorous fish which feed on marine algae and are then passed up the food chain to humans eating reef fish. We have used a unique database of reported fish poisoning cases in the South Pacific islands to investigate the relationship between fish poisoning and El Niño Southern Oscillation, a periodic disruption of global climate that is associated with marked changes in rainfall, temperatures, and ocean currents. This provides an opportunity to study the effects of a strong climate signal on a sensitive ecosystem in a region that has been less subject to local human disturbance than most others. Using a mixed ecological study design, we calculated correlations between reports of fish poisoning in individual Pacific Islands, estimates of local sea surface temperature, and the Southern Oscillation Index. Strong positive correlations between the annual incidence of fish poisoning and local warming of the sea surface were found in a group of islands which experience warming during El Niño conditions. In another group of islands, which experience cooling of the sea surface during El Niño events, there were weaker negative correlations between fish poisoning and local sea surface temperature. The results are consistent with other evidence suggesting a close interdependence of marine ecosystems and climate. Increases in ciguatera may result if the climate continues to warm as a result of the enhanced greenhouse effect. Coral reefs have been under increasing pressure from human populations in recent years; ciguatera may be a sensitive indicator of environmental disturbance in tropical marine ecosystems.