We report the case of a 7-year-old boy with Fanconi's anemia, who underwent a bone marrow transplant using an unrelated donor, and who received an inadvertent dose of cytarabine (cytosine arabinoside). The cytarabine was given by mistake 6 months following transplant. This caused excessive toxicity to many systems, including the pulmonary and renal systems. The patient recovered from the episode, but this article further highlights the acute adverse effects of cytarabine. Furthermore, it is the first report of excessive toxicity to cytarabine in a child with Fanconi's anemia. The article also highlights the problems of medication administration errors, particularly in those exquisitely sensitive to the effects of toxic drugs.