ARTICLE ABSTRACTIntravesical Bacillus Calmette-Guérin (BCG) immunotherapy preserves the bladder after resection of high-risk non–muscle-invasive bladder cancer (NMIBC). About 30% of patients experience treatment failure, which cannot be predicted a priori and carries a high risk of disease progression. We examined the in vitro tuberculin responsiveness of CD4+ T cells before BCG immunotherapy in 42 patients with high-risk NMIBC. The frequencies and functionalities of cytokine-expressing CD4+ T cells immediately before and after BCG immunotherapy induction were assessed by flow cytometry after overnight tuberculin stimulation. Tuberculin-induced secreted mediators were measured by electrochemiluminescence. We correlated the results with recurrence-free patient survival 6 months after induction. A tuberculin-induced, secreted, IL2 concentration > 250 pg/mL was the best predictor of recurrence-free survival, providing 79% sensitivity, 86% specificity (AUC = 0.852, P = 0.000), and overall correct classification in 78.6% of cases. In 50% of patients later experiencing recurrence, but not in any of the recurrence-free survivors, IL2 secretion was < 120 pg/mL. Other parameters predicting recurrence-free survival included secreted IFNγ (AUC = 0.796, P = 0.002) and the frequencies of TNF-producing (TNF+) CD4+ T cells (AUC = 0.745, P = 0.010). “Polyfunctional” CD4+ T cells (IFNγ+/IL2+/TNF+) were significantly associated with recurrence-free survival (AUC = 0.801, P = 0.002). Thus, the amount of IL2 secretion from CD4+ T cells after overnight in vitro incubation with tuberculin predicted the outcome of BCG immunotherapy. As many as half of potential BCG failures could be identified before induction therapy is begun, enabling better choices regarding treatment. Cancer Immunol Res; 6(10); 1212–9. ©2018 AACR.