BACKGROUND Methyl parathion is classed as an extremely hazardous pesticide with a rodent LD50 of 6 to 24 mg/kg. It has been banned in numerous countries, but there are few reports of acute methyl parathion poisoning. METHODS Plasma cholinesterase and acetylcholinesterase were measured in blood. Methyl parathion and the major metabolite 4-nitrophenol where measured in serum and urine. Based on the available concentration-time data, the pharmacokinetic parameters of methyl parathion were estimated for this patient. RESULTS A 29-year-old male ingested 50 to 100mL (12 to 24 g) of methyl parathion causing delayed and prolonged suppression of acetylcholinesterase but almost no clinical effects. Absorption was predicted to last for 30 hours and the bioavailability appeared to be very low. CONCLUSIONS Although it is feasible the patient ingested much less, a tenth of his alleged ingestion dose is more than the oral LD50 in rats. Methyl parathion appears to be less toxic in humans than parathion for similar amounts ingested, which is not consistent with the two pesticides having similar rodent LD50.