We describe a case of Plasmodium falciparum infection in a Comorian patient undergoing BMT. The patient's last visit to an endemic area was 1 year prior to BMT. The donor left the Comoro Islands 2 months before marrow harvesting. They had both had previous episodes of malaria and were seropositive for Plasmodium falciparum. At the time of BMT, blood smears were negative in both the donor and recipient. On day 12 post-BMT the patient was asymptomatic but a blood smear revealed 12.5% parasitemia. We consider that donors and recipients at risk pre-BMT should routinely be given specific treatment before marrow harvesting and conditioning, independent of the appearance of blood smears.