We describe a patient who, at the onset of acute myelomonocytic leukemia, presented with marked polyuria, polydipsia and laboratory findings consistent with diabetes insipidus (DI). He was treated with vasopressin (DDAVP) with a good response and concurrently induced with daunorubicin and conventional doses of cytosine arabinoside. CR was achieved. The vasopressin requirement decreased progressively, but the patient remained DDAVP-dependent after consolidation treatment. He underwent allogeneic BMT, conditioned with busulfan and cyclophosphamide. By day 15 after BMT vasopressin was no longer required and at a follow-up of 9 months the patient has no evidence of DI. In the absence of specific findings, we think it possible that he had leukemic microinfiltration of the hypothalamic-pituitary area. The drugs used for conditioning may have eradicated CNS disease.