Background: Flow cytometry in the cerebrospinal fluid (CSF) is used as an adjunct to cytology to increase the sensitivity of detecting central nervous system (CNS) lymphoma. We aim to evaluate CSF flow cytometry as a diagnostic tool for lymphoma in patients presenting with undifferentiated neurologic symptoms. Methods: We retrospectively reviewed all CSF flow cytometry samples sent in the Calgary region from 2012-2015. Clinical data, laboratory investigations, radiologic imaging studies, and pathological data were analyzed. Clinical review extended to 2 years post CSF flow cytometric testing. Results: The number of samples of CSF flow cytometry that were positive for a hematological malignancy was 43/763 (5.6%). The overall sensitivity of the test was 72.9%. A positive result was more likely to occur in patients with a prior history of a hematological malignancy or abnormal enhancement on MRI (p<0.0001). In fact, CSF flow cytometry was negative in all patients who did not have a previous hematological malignancy or abnormal enhancement on MRI (n = 247). Conclusions: CSF flow cytometry has very limited role in the screening for primary CNS lymphoma, unless a strictly endorsed testing algorithm is applied. It is, however, an invaluable tool in assessing CNS involvement in patients with previous diagnosis of hematolymphoid malignancy.