In this report, we describe a case of Neuro-Behçet's disease with early onset of bipolar mood disorder. A 53-year-old man with neuropathy including dysphasia and dyslalia developed bipolar mood disorder with anxiety, agitation, depressive mood, talkativeness, hyperkinesias, and appetite rise, and soon exhibited severe personality deterioration. Oral aphthae, cell proliferation and elevated IL-6 levels in spinal fluid, and the patient's clinical downhill course with remission and relapse in spite of good reaction to steroid preparation indicated the possibility of Neuro-Behçet's disease. Brain MRI showed clear swelling of the brain stem area, especially in the pons, in the active phase with low signal in T1-weighted images contrasting with clear high signal in T2-weighted images and FLAIR. At the time of remission, atrophy of the brain stem was shown. These findings were consistent with the view reported in recent years concerning the brain image of Neuro-Behçet's disease, which seemed to be useful for the differential diagnosis. This case manifested two outstanding clinical features. First, it showed bipolar mood swing or mixed state distinguishable from disinhibition or euphoria in deteriorated personality, which is common in this condition. A clear bipolar mood disorder has not been described until now in Neuro-Behçet's disease. Second, subclinical dysthymia or hypomanic phase occurred before clear onset of the disease. In Neuro-Behçet's disease, it is currently considered that psychiatric symptoms may appear in the early stage, but there is controversy as to whether they can precede the other symptoms. Our case indicated very early onset of psychiatric symptoms in this condition.