Juvenile myelomonocytic leukemia is a rare myeloproliferative disease in young children. While hematopoietic stem cell transplantation remains the only curative therapeutic option for most patients, children with juvenile myelomonocytic leukemia increasingly receive novel agents in phase I-II clinical trials as pre-transplant therapy or therapy for relapse after transplantation. However, response criteria or definitions of outcome for standardized evaluation of treatment effect in patients with juvenile myelomonocytic leukemia are currently lacking. Here we propose criteria to evaluate the response to the non-transplant therapy and definitions of remission status after hematopoietic stem cell transplantation. For the evaluation of non-transplant therapy, we defined 6 clinical variables (white blood cell count, platelet count, hematopoietic precursors and blasts in peripheral blood, bone marrow blast percentage, spleen size and extramedullary disease) and 3 genetic variables (cytogenetic, molecular and chimerism response) which serve to describe the heterogeneous picture of response to therapy in each individual case. It is hoped that these criteria will facilitate the comparison of results between clinical trials in juvenile myelomonocytic leukemia.