"Miss L" was an 18-year-old woman with good physical health and no personal history of psychiatric illness but a family history of bipolar affective disorder. She developed upper respiratory symptoms for 1 day, followed by a fever of 39.4°C. She was confirmed to have the influenza A virus, subtype H1N1, infection by reverse transcription-polymerase chain reaction and a viral culture of nasopharyngeal swab. The patient was prescribed oseltamivir, 75 mg twice daily, for 5 days. After the first day of treatment, she presented with elated mood, irritability, inflated self-esteem, increased talkativeness with pressure of speech, flight of ideas, and decreased need for sleep. She had delusions of "special power," and her clinical picture was compatible with mania. Blood tests and a computed tomography scan of the brain were normal. The patient did not use alcohol or illicit drugs. A urine toxicology screen was negative, and there was no clinical evidence of encephalopathy. She completed a 5-day course of oseltamivir treatment. Her manic symptoms did not subside until after 30 days of discontinuation of the drug, despite treatment with valproate, 500 mg twice daily, and risperidone, 3 mg twice daily. Because of the close temporal relationship between oseltamivir treatment and the development of manic symptoms, with no other plausible causes for neuropsychiatric manifestations, oseltamivir-induced mania was the likely diagnosis.