We reviewed records of hematopoietic cell transplantation (HCT) patients seen over the past 10 years who had head scan documentation of subdural fluid collections. A total of 17 patients were identified: 13 with allogeneic and 4 with autologous HCT (0.71% of allogeneic and 0.13% of autologous HCT patients seen in this time interval). Although less than 20% of HCT patients have lumbar puncture, 8 of the 17 subdural patients had lumbar puncture. The lumbar puncture was done 5-112 days (median 46 days) before subdural detection. Acute lymphocytic leukemia was the diagnosis in five of these eight; whereas, either acute myelogenous leukemia or myelodysplasia was the diagnosis in seven of the nine patients without lumbar puncture. In the patient group with lumbar puncture, subdurals were diagnosed earlier after HCT (median 25 days versus 5 months in the patient group without lumbar puncture) and were more often hygromas (37.5 versus 0%). These results support the suggestion of lumbar puncture or intrathecal therapy as a risk factor for subdurals. The presumptive mechanism involves lumbar cerebrospinal leak, low intracranial pressure, downward displacement of the brain, cerebrospinal fluid accumulation into the inner dural layers of the cerebral convexities (hygromas) and bleeding into these fluid collections (hematomas).