The authors report 6 cases of cerebrospinal fluid (CSF) examinations in which malignant cells were found. In 5 cases the finding was incidental: medulloblastoma (case 1); malignant melanoma (case 2); adenocarcinoma (case 3); meningitis carcinomatosa (case 4) and neuroleukaemia (case 5). In only one case neuroleukaemia was suspected before the study of the CSF (case 6). The unexpected occurrence of malignant cells in the CSF demands the pathologist to. be well acquainted with tumor cell cytology, in order to identify them providing so useful information that can decidedly influence subsequent diagnostic and therapeutic procedures. The cell collection technic developed in the authors' laboratory was considered adequate, because both inflammatory and malignant cells were securely identified. Its principle is to obtain thin air-dried smears that dry almost instantaneously on slides, in order the celular structure be preserved. After centrifuging about 5-8ml of CSF the tube is inverted so as to pour off the whole of the supernatant fluid. It is kept inverted with the operator's left hand, so that no drop of fluid can run back on to the cells. With his right hand the operator touches the bottom of the inverted tube with a Pasteur micropipette preciously made by distending a microhematocrit tube under the flame of a Bunsen burner and attached with adhesive tape to the handle of a wire loop 2mmin diameter. One little drop of the cell suspension is blown with an hematologic suction tube on a slide and spread out with the loop in the maner of a comb, taking care not to scratch over the same place twice, as it dries almost instantaneously. Only a square centimeter or so need be covered. The smear is stained with Giemsa or Leishman stain.