A prolonged type of organophosphate toxicity, previously characterized as the Intermediate Syndrome, has been recognized in 6 out of 7 prospectively studied patients poisoned by insecticide containing parathion and methyl parathion in equal proportions. The clinical characteristics included respiratory paresis, weakness in the territories of several motor cranial nerves, neck flexors and proximal limb muscles, and depressed tendon reflexes, all lasting for several days or weeks. Electromyography in the early stages disclosed diverse types of impaired neuromuscular transmission. EMG normalization preceded clinical recovery. Severe plasma butyrylcholinesterase and erythrocyte acetylcholinesterase inhibition persisted along with the occurrence of Intermediate Syndrome-related symptoms. We conclude that combined parathion and methyl parathion poisoning is more likely to induce Intermediate Syndrome than parathion poisoning alone. The mechanisms underlying this difference remain obscure. The Intermediate Syndrome shows clinical and electromyographic hallmarks of combined postsynaptic impairment of neuromuscular transmission.