1. In order to assess which laboratory parameters could be related to the prognosis of patients with acute paraquat poisoning, we reviewed the medical records of 160 patients who had ingested paraquat in an attempt at suicide. 2. Serum creatinine and potassium concentrations, arterial blood bicarbonate and base excess levels, arterial blood pH, volume of paraquat ingested and the strength of the urinary paraquat qualitative test (sodium dithionate colour reaction) on admission in the surviving patients were significantly different from those of the patients who died within 48 h of ingestion. 3. The relationship of the quantity Eq1 defined as: Eq1 = ([K+] x [HCO3-])/([Cre] x 0.088) (mEq l-1) against the interval of time after ingestion to admission (T) had a significant correlation with prognosis (P less than 0.01). Patients with Eq1 greater than (1500 - 399 X LogT) had a 90% survival rate, Eq1: (930 - 399 X LogT) less than Eq1 less than or equal to (1500 - 399 X LogT) 38% and Eq1 less than or equal to (930 - 399 X LogT) 3%, P less than 0.01.