Dyke-Davidoff-Masson syndrome, or cerebral hemiatrophy, is a pre- or perinatally acquired entity characterized by predominantly neurologic symptoms, such as seizures, facial asymmetry, contralateral hemiplegia, and mental retardation. Psychiatric symptoms are rarely reported. We report the first case of left cerebral hemiatrophy and a late onset of treatment-resistant schizoaffective disorder after a stressful life event. The patient finally responded well to clozapine. The clinical history and results from structural neuroimaging are highlighted to discuss the possible developmental bias for psychotic disorders.