Twenty cases of subdural effusion are presented occurring in infants under the age of 14 months during their convalescence from acute bacterial meningitis due to H. influenzae type b, D. pneumoniae, paracolon bacillus and Ps. aeruginosa. The incidence of these effusions must be quite high, since at least 20 out of 43 cases of meningitis under the age of 2 years had fluid in the subdural space at some time during their hospital stay. The authors believe that subdural taps should be performed on admission for diagnostic purposes whenever the patient has been treated with antibiotics before coming to the hospital, and has no organisms in the spinal fluid at the time he is first seen, as well as in all patients who give evidence of prolonged fever, focal neurologic signs, convulsions, etc., following meningitis. The trauma incident to subdural taps is probably not itself responsible for causing subdural effusions, since in most cases fluid was present at the first tap. The matter of age incidence needs investigation. There is no reason to believe that such effusions are limited to infants. The possible presence of a subdural effusion should be considered in an older patient whose convalescence from meningitis is slow, accompanied by prolonged fever, headaches and any suggestive focal neurologic signs. The authors' experience, as well as a review of the older literature, indicates that from the confusing clinical entities known as subdural hygroma, pachymeningitis hemorrhagica interna, etc., one group can be singled out as occurring on the basis of an underlying infection. Its frequent occurrence in recent years is owing to the high survival rate in bacterial meningitis. The actual pathogenesis of this lesion; the incidence of loculated fluid; the incidence of "membranes"; the possible influence of different treatment regimens on the occurrence of this syndrome; the possibility suggested by analogy with "pleuropneumonia" that subdural effusions, when infected, may serve as foci for later recurrences of meningitis; the possible influence of early recognition and treatment of subdural effusions on the ultimate prognosis in meningitis are matters for the future to determine.