Low-level contamination events are common but often neglected because they may not immediately harm the environment or wildlife. We suggest that even seemingly benign oil spills may cause severe damage for wildlife and we offer a rapid assessment tool for the health status of a wildlife population that is potentially affected by a spill. We studied two island populations of seagoing lizards, marine iguanas (Amblyrhynchus cristatus), for 20 years before a low-level oil spill hit the shores of the Galapagos archipelago in January 2001. Under natural conditions, the main cause of mortality for marine iguanas is starvation caused by a decline in their staple food, marine algae. Starvation induces a strong stress response during which the hormone corticosterone is secreted. When iguanas of one island were exposed to low-level oil contamination of their food after the tanker "Jessica" grounded, they also showed a strong stress response, allowing us to predict high mortality rates for oiled individuals. Indeed, one year after the spill 62% of the iguanas on the oiled island had died, while none of the control iguanas on the non-oiled island died. We hypothesize that oiled iguanas died from starvation because their digestive gut bacteria were killed by oil residues in their algae food. Although the exact causation of oil-induced mortality is unknown and should be investigated further, we suggest that the corticosterone-stress response is a powerful predictor for the rapid assessment of wildlife health. Low-level oil contamination appears to be a serious threat to wildlife. (C) 2002 Elsevier Science Ltd. All rights reserved.