Relationship between in vitro PRD resistance (cut-off values for sensitivity and resistance 0.1 and 150 μg/mL, respectively) and long-term clinical outcome after combination chemotherapy in childhood ALL, within (A) the total group of clinical good PRD responders (15 highly sensitive, 79 intermediately sensitive, and 30 resistant to PRD in vitro), and (B) the group of clinical poor PRD responders (none highly sensitive, six intermediately sensitive, and six resistant to PRD in vitro). The numbers in the figures along the x-axis indicate the patients at risk for a relapse at the different time points.