Trisomy 10 as the only chromosome aberration is a rare phenomenon in malignant and premalignant hemopoietic disorders. We describe 7 new cases and have found another 12 in the literature. It appears that, whereas adult patients have myeloid disorders (acute myeloid leukemia, myeloproliferative, or myelodysplastic syndromes), in children the diagnosis is lymphocytic leukemia or lymphoma. The median survival was 122 months in the total material. Age above 60 years proved to be a significant adverse factor (median survival only 5 months; p = 0.003). None of the other clinical, cytogenetic, or hematological variables were of demonstrable prognostic importance. In contrast with the larger trisomy 10 clones, those of limited size were associated with nonleukemic diagnoses, normal or slightly elevated leukocyte counts, and few or no circulating blasts. This may suggest that expansion of the trisomy 10 clone is associated with clinical and hematological progression.