Disorder of train of thought is one symptom of the formal thought disturbance. There are several types of disorder of thought train, such as loosing of association, flight of idea, incoherence, inhibition of thought, blocking of thought, circumstantial thinking and perseveration. Although these symptoms are characteristics of functional mental disorder, they can be observed as one of the symptoms of the dementia, delirium and disturbance of consciousness in the elderly. If the elderly patients show these symptoms with psychosis, mania and depression, the organic or drug induced mental disorder should be excluded. Patients with aphasia or parkinsonism also should be paid attention to examine train of thought, so they often seem to have inhibition of thought and blocking of thought.