Morphologic detection of blast cells in the cerebrospinal fluid at diagnosis of adult acute lymphoblastic leukemia appears to be associated with adverse prognosis. 
Appropriate treatment of central nervous system (CNS) involvement in adult acute lymphoblastic leukemia (ALL) is important for patient prognosis, but the diagnostic criteria of CNS involvement has not been established.  The significance of blast cells in the cerebrospinal fluid (CSF) at diagnosis was evaluated in 81 adults newly diagnosed with ALL.  Patients with unequivocal morphologic evidence of lymphoblasts in the cytocentrifuged CSF slide were considered to have CNS involvement regardless of the leukocyte count.  The outcomes of the patients were analyzed.  Four of the 81 patients (5%) had detectable blast cells, and three of these four patients had less than five leukocytes/μL of CSF.  One-year event-free survival (EFS) was 25.0% and 53.2% (P = 0.008) and overall survival (OS) was 50.0% and 68.8% (P = 0.001) in patients with and without CNS involvement, respectively.  CNS involvement had prognostic impact on EFS (P = 0.047) and OS (P = 0.009) after adjusting for sex, age, leukocyte count, Philadelphia chromosome status, and immunophenotype.  This study suggests that morphologic detection of blast cells in the CSF at diagnosis of adult ALL, regardless of the leukocyte count, is associated with adverse prognosis.