AIM: To clarify laboratory guidelines for cerebrospinal fluid (CSF) cytology. METHODS: Clinical and pathological data relating to 54 patients with cytologically malignant cells in the CSF were reviewed, together with CSF cell counts and protein measurements for 29 patients. Utilising this data, criteria were established for CSF cytology and validated by review of 100 patients in whom CSF cytology had not been carried out on the basis of these criteria. RESULTS: There was only one false positive diagnosis of malignancy on the basis of CSF cytology. All patients with malignant cells in the CSF fulfilled at least one of the following criteria: clinically known or suspected malignancy; raised cell count; raised protein concentration. In none of the 100 patients, in whom cytology was not performed, was the diagnosis of malignant meningitis missed. CONCLUSION: Cytology should be performed on CSF specimens from all patients with known, or suspected, malignancy, but in other cases, only if the cell count or protein concentration, or both, is raised.