Aplastic anemia is a rare, serious disease characterized by hypocellular bone marrow and pancytopenia in the peripheral blood. Most cases are acquired, idiopathic, and without gross cytogenetic abnormalities. A few chromosome abnormalities have recurred among a small subset of patients, most commonly trisomy 8 and monosomy 7. Some of these chromosome abnormalities have prognostic and therapeutic significance, although for most the clinical relevance is not known. We present the case of a 40-year-old man with idiopathic severe aplastic anemia in bone marrow cells with trisomy of the whole long arm of chromosome 1 due to an unbalanced translocation between chromosomes 1 and 15 at breakpoints of q10 and 15q10. This clonal abnormality (which, to our knowledge, has not been previously reported in a patient with aplastic anemia) suggests that genes on 1q may be involved in marrow aplasia.