Progressive multifocal leukoencephalopathy (PML) is a rare fatal complication of allogeneic bone marrow transplantation (BMT) resulting from chronic immunosuppression and impaired cellular immunity. This report discusses 2 cases of PML in patients with acute myeloid leukemia after allogeneic BMT. Diagnosis was made based on characteristic brain MRI findings and positive PCR results for John Cunningham virus in the cerebrospinal fluid. Unfortunately, therapeutic options are limited and nearly always result in terminal outcomes. Although immunosuppression is an unavoidable risk of allogeneic BMT, these cases highlight a rare, yet fatal, consequence of prolonged T-cell lymphopenia and impaired cellular immunity after allogeneic BMT in this patient population.