Thirty cases of tuberculous encephalopathy with and without meningitis are described with delineation of the different clinical types. The following were the main clinical categories in this series: (1) tuberculous meningitis with encephalopathy in 15 cases (1 fulminating, 2 acute, 6 subacute and 6 chronic); (2) serous tuberculous meningitis with encephalopathy in 8 cases (4 subacute and 4 chronic); (3) tuberculous encephalopathy without meningitis in 6 cases (3 fulminating, 1 acute, 1 subacute and 1 chronic); and (4) acute haemorrhagic encephalopathy in 1 case. The main clinical features of diffuse brain involvement were impaired consciousness, convulsions, involuntary movements, decerebrate and decorticate postures, paralysis, pyramidal, extrapyramidal and cerebellar signs. In all cases there was evidence of tuberculosis. There was pathological evidence of miliary tuberculosis in 7, intracranial tuberculosis in 15, intrathoracic in 11, and abdominal in 8. Acid-fast bacilli were detected in the cerebrospinal fluid in 1 case and in the meningeal exudate in another. Radiological evidence of the disease was present in 15 cases. A family history of tuberculosis was obtained in 6 cases. All the 4 cases with a fulminating illness and 3 acute cases died; 6 of the 7 with a subacute illness died; and 4 of the 14 with a chronic illness died. Of the 13 who survived, 4 recovered fully, while 9 showed mild or severe sequelae, in 2 of whom the illness ran a very prolonged course. The neuropathological changes were those of an oedematous encephalopathy with diffuse or patchy myelin loss in the white matter, commensurate glial reactions, occasional haemorrhagic and glial nodules, and with moderate to severe diffuse changes in the grey matter, especially the cortex. Pathogenetic mechanisms are discussed and the role of inflammatory, vascular and allergic factors are considered. The latter appear more significant and sensitivity to tuberculoprotein or to brain itself may be responsible.