Low-dose interferon-alpha is a standard therapy for hepatitis C. Psychotic disorders have been described as a rare complication of such treatments that resolve with its termination. Here, we present a patient without significant risk factors for interferon-alpha-induced serious mental disorders who developed a psychotic disorder with a cognitive impairment achieving the level of dementia after seven months of interferon-alpha therapy. The disturbances have persisted for three years despite cessation of interferon and introduction of antipsychotic treatment. The possibility of severe neuropsychiatric adverse effects of interferon-alpha therapy in a susceptible individual may necessitate regular psychiatric consultations during the treatment.