We describe here two cases of myelodysplastic syndrome (MDS) with a novel unbalanced translocation der(5;19)(p10;q10). Both patients had complex karyotypes including der(5;19) accompanied by an extra chromosome 19, resulting in deletion of the whole long arm of chromosome 5. Furthermore, these patients presented several common clinical and hematological characteristics: MDS subtypes as refractory anemia with excess of blasts (RAEB)-1 or RAEB-2, marked anemia and thrombocytopenia without neutropenia, leukoerythroblastosis, trilineage dysplasia with prominent dyserythropoiesis, CD7 expression in blasts, and association with abnormalities of chromosomes 6, 17 and 18. These findings indicate that der(5;19)(p10;q10) may play a crucial role in the pathogenesis of high-risk MDS as a rare but recurrent translocation.