Diabetes insipidus together with acute myelogenous leukemia has rarely been seen. Still rarer is the occurrence of monosomy 7 with the two diseases (only six cases reported). A patient who had diabetes insipidus develop before the diagnosis of acute myelogenous leukemia was found at karyotyping to have monosomy 7. Although a specific mechanism whereby monosomy 7 would cause diabetes insipidus has been proposed, some have suggested that monosomy 7 may have its effect by altering cell wall membranes. Others have suggested that acute myelogenous leukemia causes diabetes insipidus by causing infiltrates in the hypothalamus or posterior lobe of the pituitary gland. Magnetic resonance imaging of the patient's brain showed no abnormalities of the hypothalamus or pituitary gland. Lumbar puncture revealed no leukocytes in the cerebrospinal fluid. The authors believe that the cause of diabetes insipidus can be explained in patients with acute myelogenous leukemia by checking for monosomy 7 during karyotyping. Because karyotyping is now more frequently performed in evaluation of patients for chemotherapy or bone marrow transplantation, genetic abnormalities such as monosomy 7 will become increasingly apparent.