Acetaminophen overdose was studied in 416 patients as part of a nationwide multiclinic open study. Twenty-seven percent of the patients were toxic by plasma acetaminophen level. The 12- to 21-year-old age group had the highest incidence of toxic blood levels (47%). Children under age 5 had the mildest toxicity of any age group; only two of 16 patients had toxic blood levels. No relationship to subsequent Reye's syndrome could be demonstrated. Acetylcysteine is a safe, effective treatment if administered within the first ten hours; it has some efficacy with no toxicity if used after ten hours. Acetylcysteine has advantages over both cysteamine and methionine. All patients in this study recovered with no sequelae.