Acetaminophen (paracetamol) is used throughout the world for pain relief and antipyresis in both children and adults. In many countries, it can be purchased without a medical prescription and it is also a common component of a number of over-the-counter remedies for colds, influenza and the like. Fasting, malnutrition and use of alcohol and/or other drugs are thought to play causal roles in hepatotoxicity associated with recommended doses of acetaminophen although liver injury provoked by therapeutic doses has also been observed in the absence of these factors. We describe two patients who experienced subclinical hepatotoxic reactions after taking acetaminophen at therapeutic doses. The results of an antipyrine metabolism test suggest the presence of constitutional hyperactivity of the cytochrome P450-dependent mixed function oxidative system in both patients. We hypothesize that the latter contributed to the hepatotoxicity and that it may play a role in idiosyncratic reactions to this drug.