A patient presented without symptoms 30 minutes after ingesting acetonitrile, also known as methylacyanide. He had prompt gastric lavage and activated charcoal administration. Hours later, the onset of clinical toxicity was heralded by mental status abnormalities and vomiting prior to a generalized seizure. Following administration of sodium thiosulfate, the patient made an uneventful recovery. A blood cyanide level drawn shortly after presentation, but reported after the patient had been discharged, documented significant exposure. During hospitalization, cyanide toxicity was inferred from the history of ingestion of acetonitrile, plus a significant absence of venous blood hemoglobin desaturation. Because even small amounts can be harmful and toxicity is delayed, all acetonitrile ingestions should be presumed dangerous. Patients should be observed and repeatedly evaluated for at least 24 hours. In the absence of cyanide level determinations, lethargy, vomiting, seizures, and the lack of normal venous blood hemoglobin desaturation are clues to cyanide toxicity.