Among 56,709 cytogenetic studies performed during a 15-year period at the Mayo Clinic, 25 cases of t(5;12) were identified. Among 11 patients with available clinical information, 4 had myelodysplastic syndrome, 2 had acute myelocytic leukemia, 2 had myelofibrosis with myeloid metaplasia (MMM), 2 had atypical chronic myelocytic disorder (ACMD), and 1 had chronic myelomonocytic leukemia (CMMoL). The 5q arm was involved in all patients and the 12p arm in only two patients [ACMD,t(5;12)(q33;p13) and MMM,t(5;12)(q11.2;p11.2)], both of whom had eosinophilia and monocytosis. These two features were present in only two other patients [CMMoL,t(5;12)(q35;q24.1) and ACMD,t(5;12)(q31;q24.1)]. The t(5;12) is a rare, myelocytic-exclusive cytogenetic abnormality with a breakpoint-specific association with eosinophilia or monocytosis.