Often, symptoms of Reye's syndrome appear during recovery from a viral infection, such as the flu (influenza) or chickenpox (varicella). The symptoms develop rapidly over several hours to a day or two.

The first symptoms may include:

• Sudden onset of retching or vomiting that is not clearly due to stomach flu.
• Sluggishness, lack of energy, and loss of interest in surroundings.
• Strange behavior, such as staring, irritability, personality change, and slurred speech.
• Drowsiness that may lead to severe sleepiness (stupor).

As brain damage progresses, other symptoms may develop, including:

• Confusion and inability to identify whereabouts or family members or to answer simple questions (disorientation).
• Rapid, deep breathing (hyperventilation).
• Aggressive behavior, such as hitting others without reason (combativeness).
• Seizures and abnormal body positioning, and coma.

If Reye's syndrome is not recognized and treated promptly, death can occur.

Before a diagnosis of Reye's syndrome is made, other conditions with similar symptoms may be considered, since Reye's syndrome is so rare.