A 59-year-old man was a potential donor for allogeneic hematopoietic stem cell transplantation and was found to be healthy but slightly polycythemic. The bone marrow was morphologically normal, but karyotyping of bone marrow cells showed t(6;10)(q27;q22) in 7 of 20 metaphases analyzed by G-banding and only the t(6;10) abnormality in 3 of 5 metaphases analyzed by the spectral karyotyping method. G-banding analysis of peripheral blood mononuclear cells cultured with phytohemagglutinin for 72 hours showed a normal karyotype in all 20 metaphases analyzed.These findings suggest clonal expansion with t(6;10)(q27;q22) in the bone marrow of this individual. He was determined to be ineligible for donation. A coordinated nationwide work-up for older donors is necessary to ensure high-quality standards.