We report a case of anaplastic large cell lymphoma (ALCL) with central nervous system relapse in an 11-year-old boy. The relapse was suspected on morphologic examination of the cytospin preparations of the cerebrospinal fluid (CSF) with a WBC of 10 cells/μl. CSF relapse was confirmed using immunohistochemistry (IHC), fluorescence in situ hybridization (FISH), and reverse transcriptase-polymerase chain reaction (RT-PCR) for abnormal ALK expression or gene structure. The patient developed large intracranial metastases, despite systemic, and intrathecal chemotherapy. This case demonstrates the feasibility of detecting ALCL in paucicellular CSF specimens and suggests that even low CSF involvement can herald massive parenchymal disease. © 2004 Wiley-Liss, Inc.