During the past two years, our laboratory has introduced a semiquantitative membrane filter approach to cerebrospinal fluid (CSF) cytology. Five hundred and thirty-five CSF specimens, averaging 3 ml in volume, were placed over 25-mm diameter Millipore filters (5-mu pore size) and stained using a modified Papanicolaou method. Each microscopic evaluation consisted of a cell count per milliliter, a 100-cell differential, and a diagnostic or interpretive statement. Seventy-four (13.8%) positive, 17 (3.2%) atypical, 439 (82%) negative, and 4 (1%) unsatisfactory specimens were evaluated. Seventy-four percent of the positive or atypical CSF specimens had mononuclear cell counts within the medically accepted normal range of five mononuclear cells per cubic millimeter (chamber count method) or 5,000 cells per millimeter (cytologic method). The small-filter technique offered advantages because it maximized cell retrieval, localized cells, and shortened the time required for microscopic evaluation.