The tumor microenvironment can protect cancer cells from conventional anticancer therapies. Thus, targeting these protective mechanisms could eradicate therapy-resistant cancer cells and improve outcomes. Interleukin-6 (IL-6) provides extrinsic protection for several solid tumors and multiple myeloma. In pediatric acute myeloid leukemia (AML), IL-6-induced STAT3 signaling frequently becomes stronger at relapse, and increases in IL-6-induced STAT3 activity are associated with inferior survival after relapse. These findings suggested that the IL-6-induced STAT3 pathway may promote chemotherapy resistance and disease progression. Thus, we investigated the dysregulation of IL-6 levels in the bone marrow niche in pediatric patients with AML and the association between IL-6 levels and outcome. We measured levels of over 40 cytokines and growth factors in plasma from diagnostic bone marrow aspirates of 45 pediatric AML patients and 7 healthy sibling controls. Of the measured cytokines, only IL-6 levels were associated with event-free survival. Importantly, the effect of elevated IL-6 was most striking among children classified as having a low risk of relapse. In these patients, 5-year event-free survival was 82.5% ± 11% for patients with low IL-6 levels at diagnosis (n = 14) compared with 17.3% ± 11% for patients with elevated IL-6 (n = 13, log-rank P = .0003). In vitro, exogenous IL-6 reduced mitoxantrone-induced apoptosis in cell lines and primary pediatric AML samples. These results suggest that IL-6 levels at diagnosis could be used to help identify children at high risk of relapse, particularly those who are otherwise classified as low risk by current algorithms. Moreover, the IL-6 pathway could represent a target for overcoming environment-mediated chemotherapy resistance.