Although it is often claimed that the presence of a single polymorphonuclear leukocyte (PMN) in the cerebrospinal fluid (CSF) is abnormal, recently some have suggested that a few PMNs are occasionally present in cytocentrifuged differential cell counts of normal CSF. We examined 225 consecutive normal CSF specimens to determine how frequently PMNs occur in normal CSF and to identify factors associated with the presence of PMNs. One or more PMNs were present in 73 cases (32%). The number of CSF PMNs was strongly correlated with the degree of CSF blood contamination and the hematologic PMN count. Of the 163 specimens having 25 red blood cells or less per cubic millimeter, only eight (5%) had three or more PMNs, and these outliers had abnormally high hematologic PMN counts. Of the 36 specimens having 100 red blood cells or more per cubic millimeter, 17 (47%) had six or more PMNs. We conclude that the number of PMNs found on cytocentrifuged differential cell counts is highly dependent on the degree of CSF blood contamination and the patient's hematologic PMN count and that even minimal blood contamination can result in the presence of one to two PMNs in normal CSF.