Cerebrospinal fluid (CSF) pleocytosis after seizure activity has been anecdotally reported for many years, but it has not been well documented. We reviewed the records of all adult patients admitted to Grady Memorial Hospital from November 1979 through October 1980 with the diagnosis of seizure. Of 102 patients whose CSF was examined, 35 (34%) had pleocytosis; in 31 (30%) there was no explanation for the pleocytosis despite laboratory and radiologic tests to rule out established causes. For those patients without an identifiable cause of pleocytosis the mean number of white cells was 72/cu mm with a median of 10 and a range from 3 to 464. A predominance of polymorphonuclear leukocytes (PMNs) was found in 57% of the initial CSF examinations. Eighty-six percent of patients with seizures due to ethanol withdrawal had a PMN predominance in their CSF, and 88% of patients with seizures due to a recent or remote cerebrovascular accident had a mononuclear cell predominance. The pleocytosis was usually transient; normalization of the CSF was associated with the rapid recovery of the patient. We conclude that an abnormal CSF leukocyte count may be entirely attributable to seizure activity, although the mechanism is unknown. Before assigning this cause, however, a thorough search is imperative to rule out treatable disorders that may cause CSF pleocytosis.