1 In humans, the accidental or voluntary ingestion of paraquat, a non selective contact herbicide, is often lethal. Paraquat induces renal failure amongst other effects.2 In this study we test the value of an index of the rate of increase in plasma creatinine over a 5 h period (dCreat/dt) to predict the outcome in intoxicated patients. These results are compared to previously published indices of severity of the paraquat poisoning. The proposed index is reliable, within the capability of any laboratory and will indicate the prognosis for poisoned patients within 6 h of admission to hospital.