Some clinical and pathological features of carcinomatosis of the meninges are reviewed along with a report of four cases. This condition usually presents in middle age as a subacute meningitis with cranial nerve involvement, but the diagnostic importance of the various mental disturbances which may be encountered early in its course are noted. The acute or subacute course may reflect a widespread mechanical interference with normal cerebral metabolism, a notion which is supported by recent clinical measurements in these patients of the rate of glucose transport across the blood-brain barrier. It is probable that the route taken by tumour cells to reach the meningeal spaces is a relatively unimportant factor in determining this pattern of growth and that the intrinsic growth characteristics of the primary tumour, its nutritional needs, and gravity probably play the major roles in production of this unusual type of secondary invasion. Greater therapeutic use of irradiation for these patients is encouraged.