Serial studies were done on cerebrospinal fluid (CSF) from a patient with sarcoidosis involving the meninges. Initially when the disease was active, the CSF protein was increased and glucose decreased. The number of cells in the CSF was moderately increased, and many mononuclear cells were present. Cytologic studies of the CSF showed many normal and some atypical lymphocytes. Immunochemical studies showed that most of these lymphocytes were T cells, with T-helper cells predominating over T-suppressor cells by a ratio of 3.92; B-lymphocytes were polyclonal. Subsequent studies of the CSF over the following three and one-half years showed that the protein and glucose content and the cell counts in the fluid did not correlate well with the activity of the disease. The number of atypical lymphocytes seemed to be a more useful marker of disease activity in the patient. Cytologic studies, when interpreted within the context of other CSF and clinical findings, are useful for the assessment of patients with sarcoidosis involving the meninges.