This article presents a classification system of spontaneous cerebrospinal fluid middle ear effusion to facilitate early diagnosis and lesion localization for surgical correction. Thirty-eight articles containing case histories of 45 patients with spontaneous cerebrospinal fluid middle ear effusion reported in the English literature from 1913 to 1983 were reviewed. Four additional personal cases, representing the largest single series, are described in detail. Classification of these cases into the following three types has been possible: (type I, abnormal connections through the otic capsule; type II, abnormal connections adjacent to the otic capsule; and type III, abnormal connections distant from the otic capsule. The diagnostic and therapeutic implications of each type are unique. Age, auditory and vestibular test results, and computerized or hypocycloidal tomography differentiate the three.