Dengue virus infections are a frequent cause of morbidity and mortality in certain endemic areas of the world but it has never been reported to occur in a bone marrow transplant patient. We describe a pediatric patient with ALL on her first bone marrow relapse undergoing allogeneic bone marrow transplant who developed sepsis and irreversible shock that on postmortem autopsy proved to be secondary to dengue virus infection Type 4. A 6 years old female with ALL underwent an allogeneic bone marrow transplant from her sibling (6/6 match) on Nov 10, 1994. She received TBI (1200 rads) and ARA-C (3gm/m2 per dose per 12 doses) as preparative regimen. She developed generalized skin erythema on day +5 followed by fever on day +6. The following days were characterized by continued fever spikes, severe mucositis, persistence of the erythematous skin rash (first degree burn like) and gross hematuria in spite of broad spectrum antibiotic coverage. Blood cultures were reported negative. She developed irreversible shock and finally died on day +11 post-transplant. At autopsy (Nov 22, 1994), Dengue virus type 4 was isolated from blood, ascitic fluid, liver and spleen by routine viral culture at San Juan C.D.C. Dengue Branch. Results were confirmed by polymerase chain reaction. Her donor also developed low grade fever the day after bone marrow donation. Further blood testing on donor (Dec 2, 1994) showed positive IgM antibodies against Dengue virus Type 4. The recipient viral cultures matches with donor acute titers for Dengue Type 4, suggesting infection through bone marrow transplant. Dengue virus infection may cause significant morbidity and mortality to blood and bone marrow recipients in endemic areas of the world.