Patient outcomes in T-cell acute lymphoblastic leukemia and lymphoblastic lymphoma (T-ALL/LBL) can be difficult to predict. Simonin et al report deletions and mutations in the gene encoding the transcription factor IKAROS among 1260 children and adults with immature T-ALL/LBL, defining its frequency and, importantly, its association with poor outcome in multivariate models. Pathogenic alterations in IKZF1 now can be added to minimal residual disease detection and the established 4-gene oncogenetic classifier to better predict poor outcomes of T-ALL/LBL.