Acetaminophen is an effective mild analgesic and antipyretic agent. In double-blind, controlled experimental pain studies of short duration, acetaminophen is superior to placebo and produces analgesia comparable to that produced by aspirin. The frequency of adverse reactions to therapeutic doses of acetaminophen is low, as is that of aspirin. Overdosage with acetaminophen, however, may result in irreversible hepatotoxicity. Since clinical manifestations of intoxication can be of slow onset, physicians may tend to delay initiation of definitive therapy. Intravenous cysteamine, and possibly oral methionine, appear to be effective in preventing hepatotoxicity if they are administered with 10 h of drug ingestion. Physicians should be aware of the potential danger of acetaminophen overdosage and alerted to its clinical manifestations.