AbstractHepatitis B surface antigen (HBsAg) was detected in the cerebrospinal fluid (CSF) of five of nine leukemia patients with HBsAg‐positive sera. These five CSF samples were shown to be free of occult blood using a sensitive hemoglobin extraction technique. The presence of HBsAg in CSF was unrelated to cranial irradiation, prior lumbar punctures, or current chemotherapy. The findings indicate that many blood‐free CSF specimens from HBsAg‐positive patients, as well as all blood‐contaminated specimens, contain detectable HBsAg. This suggests that CSF from patients who are HBsAg‐positive should be handled with care, as it may be capable of trasmitting hepatitis B to health care personnel caring for these patients and laboratory personnel handling their specimens.