We report a patient in whom intoxication with a commercial preparation of malathion led to a severe acute cholinergic crisis, protracted cardiac arrhythmias, an acute respiratory distress syndrome, and an acute sensorimotor axonal polyneuropathy. The occurrence of the polyneuropathy was associated with a novel malathion transformation product, S-(1-ethoxycarbonyl, 2-isopropoxycarbonyl) ethyl 0,0-dimethyl phosphorodithioate (isopropylmalathion), an impurity present in the pesticide residue. The role of isopropylmalathion in the pathophysiology of the polyneuropathy remains to be determined.