During a two-year period 170 children were treated for leukemia. Forty-seven per cent had clinical and cytologic evidence of central nervous system involvement. Of these, 38 patients had repeated cytologic examinations of the spinal fluid and were suitable for a study of changes that occurred during the course of their disease and treatment. A total of 112 cytologyc specimens from the 38 patients over a period of 8 to 60 months was reviewed. There was accurate correlation between cytologic evidence of leukemia in the spinal fluid, clinical signs and symptoms of central nervous system disease, and autopsy evidence of meningeal leukemia, though the number of cells present did not predict the severity of clinical disease or extent of meningeal infiltrate. Almost every patient with leukemic cells in the spinal fluid had a corresponding leukemia infiltrate in the bone marrow, but not in peripheral blood. The differential diagnosis of leukemia in spinal fluid cytology is discussed and the clinical response to intrathecal chemotherapy correlated with cytologic findings. Serial cytologic observations during treatment, and the phenomenon of autonomous CNS leukemia, are discussed in terms of cell cycle physiology and chemotherapy. The results stress the usefulness of spinal fluid cytology as a screening procedure in asymptomatic patients with leukemia and are of interest in regard to the pathophysiology of leukemia of the central nervous system.