The present study looked for any associations between in vitro drug sensitivity and clinical outcome in pediatric acute lymphoblastic leukemia (ALL) with the standard drugs used for leukemia therapy. A total of 72 samples were analyzed. In vitro sensitivity to drugs was tested by a methylthiazol-tetrazolium assay in 6 serial fold dilutions. Apoptosis was determined by TUNEL assay and apoptotic index was calculated for each sample. Patients sensitive to prednisone, asparginase, vincristine, and 6-mercapto purine had higher overall survival compared to patients whose tumor cells were resistant to these drugs (p < .01). For the other drugs tested, overall survival did not vary from that of the resistant patients. For doxorubicin, asparginase, vincristine, prednisone combination sensitivity, there was a significant worsening of prognosis from the extremely sensitive patients through an intermediate sensitive group to a most resistant group. The present study thus shows that in vitro drug-sensitivity testing provides significant prognostic information in childhood ALL.