Traditional clinical trials rely on the random allocation of patients into control and treatment groups, a model that reduces bias but fails to account for individual biological heterogeneity, which is particularly relevant in pediatric acute lymphoblastic leukemia (ALL). We propose that integrating functional drug screening into clinical trial design can transform this paradigm by enabling biologically informed randomization. In this model, primary leukemic cells from each patient are tested ex vivo against candidate drugs prior to clinical trial enrollment, allowing the identification of individuals who are functionally sensitive to specific therapies. This strategy enables rational patient selection, maximizing therapeutic benefit while minimizing unnecessary exposure to ineffective treatments. To test this hypothesis, we performed ex vivo functional screening using the ELDA platform (Ex Vivo Leukemia Drug Advisor), which consists of primary ALL cells cultured in a 3D collagen matrix with bone marrow stromal co-culture, followed by time-lapse brightfield microscopy imaging every 30 minutes for 72 hours and automated viability analysis based on membrane dynamics. A total of 114 primary pediatric ALL samples were screened with docetaxel, vincristine, clofarabine, and cytarabine. Results showed that only ∼10% of samples were sensitive to docetaxel. These sensitive samples were subsequently validated in vivo using PDX (patient-derived xenograft) mouse models, in which immunocompromised mice were engrafted with leukemic cells and treated with docetaxel, vincristine, or control. Leukemia progression was monitored by peripheral blood flow cytometry (human CD45/mouse CD45). The docetaxel-treated group showed the longest survival, outperforming vincristine, a drug of standard pediatric ALL therapy, demonstrating that docetaxel can be therapeutically superior in a functionally selected subgroup. Similarly, we identified patients who were functionally resistant to cytarabine (standard chemotherapy) but sensitive to clofarabine. In vivo validation in PDX models confirmed improved survival in the clofarabine-treated group compared to the cytarabine-treated group, consistent with ex vivo functional predictions. Previous clinical trials evaluating docetaxel or the substitution of cytarabine with clofarabine in pediatric ALL failed to demonstrate population-level benefit, leading to the rejection of these strategies. Our data suggest that these negative outcomes may reflect the lack of functional patient stratification, whereby true responder subpopulations are diluted within unselected cohorts. We conclude that ex vivo functional drug screening enables the identification of biologically defined responder subgroups, redefining clinical trial randomization as a biologically driven process. This approach has the potential to reshape clinical trial design, optimize drug development, and establish a truly functional precision medicine framework in pediatric oncology.