Chromosomal anomalies involving region 3q21q26 have been reported associated with thrombocytosis in various hematological malignancies, such as chronic myeloid leukemia, myelodysplastic syndrome, and acute myeloid leukemia (AML). Recent reports described the association of central diabetes insipidus with AML and chromosomal anomalies involving region 3q21q26. We review the database in our institution and report five cases of inv(3)(q21q26) in consecutive cytogenetic studies of AML and myelodysplastic syndromes from 1992 to 2000, two cases presented as sole abnormalities and three cases were associated with monosomy 7. Only one case was associated with central diabetes insipidus. The literature of 3q21q26 syndrome and central diabetes insipidus in myeloid leukemia is reviewed.