Evidence in support of the concept of direct involvement of the central nervous system by measles virus may be summarized as follows: (1) The hall-marks of the measles virus, typical cytoplasmic or nuclear inclusions bodies, or both, have been demonstrated in 15 of 20 fatal cases. (2) Small multinucleated giant cells have been recorded in five patients. (3) Evidence that the neurological signs and symptoms may precede the rash of measles is clear from the literature and is supported by the first two cases presented in this study. (4) Nearly all of the patients had evidence of the classic pathological changes of perivascular infiltration, accompanied in nearly all instances by de-myelination. When the lesions are followed over a period of time, a fusion of the perivenous areas may take place which in the long standing cases tends to form large plaques of sclerosis as demonstrated in cases 15 and 20. Two of the patients who died received [gamma]-globulin several days prior to the onset of neurological symptoms and death. One patient with leukemia received several transfusions and steroid therapy prior to measles, and died eight weeks following the rash with abundant evidence of measles pneumonia and encephalitis at autopsy. Much further study will be required before any firm conclusions can be drawn. The final answer will undoubtedly depend on the unequivocal demonstration of the measles virus in the central nervous system in measles encephalitis.