We identified 18 patients with an isolated bone marrow clonal chromosomal abnormality, trisomy 15 (with or without -Y), who did not have any morphologic or clinical features of hematologic disease at initial examination. All but 1 patient was older than 65 years. Fourteen patients had underlying nonhematologic, chronic diseases. Four patients had histories of hematologic malignancies. Eleven of the patients also had a -Y. All but 1 case had a marrow cellularity of less than 50%. Extensive morphologic review of the specimens showed no features of myelodysplastic syndrome (MDS). Clinical follow-up was available for all patients and ranged from 1 month to 16.3 years (median, 4.0 years). No subsequent clinical or hematologic manifestations of MDS were identified in any of these patients. Clonal trisomy 15 in isolation or in combination with -Y is an uncommon cytogenetic finding that does not seem to be associated with definitive morphologic or clinical features of MDS or any other malignant process.