We present the case of a 14-year-old Hispanic boy with a 6-month history of a psychotic disorder necessitating several hospitalizations who was incidentally found to have multiple sclerosis with no physical findings. Neuropsychological assessment has revealed impairments in word-finding, bilateral fine motor skills, and attention. Imaging and laboratory studies have supported the diagnosis of multiple sclerosis. Steroid and immunomodulating therapy has not significantly affected psychiatric symptoms. He has had poor response to psychotropic medications as well. We discuss the implications of multiple sclerosis as the cause of this patient's psychiatric illness. (Contains 3 figures.)