We evaluated 322 patients with ALL younger than 22 years at diagnosis who received their initial therapy at Seattle Children's Hospital (SCH) between January 1997 and December 2005. We excluded patients who received their initial month of treatment at another hospital. Two patients with significant comorbidities received noncurative palliative treatment and were excluded from analysis. All other patients were offered therapeutic clinical trial participation if an open study was available. If patients declined participation or no study was available, all patients received therapy according to strictly defined institutional standard therapies based on the most recent clinical trial results. During the study period, clinical trials available for enrollment included the following Children's Oncology Group or Children's Cancer Group legacy protocols: 1952,11 1953,12 1961,13 1962,14 1991,15 AALL00P2,16 AALL0232, and AALL0331. We analyzed the following variables: participation in an ALL therapeutic study, sex, race, age at diagnosis, leukemia immunophenotype, National Cancer Institute risk group (standard risk, high risk, or infant),17 home state, and distance of primary residence to SCH. Seattle Children's Hospital is the regional referral center for Washington, Alaska, Montana, Idaho, and Wyoming and the largest pediatric oncology program in the region. Statistical analysis of event-free survival (EFS) was performed using the Kaplan-Meier method for calculating survival curves and 95% confidence intervals (CIs).18 Event-free survival was defined as the time from initial diagnosis to either leukemia recurrence or death from any cause. Patients who had not developed relapse or death were censored at the date of last contact. Differences in EFS among groups defined by patient or treatment characteristics were analyzed using the log-rank test.19 The EFS data were analyzed as of March 15, 2009, using SPSS for Windows version 13.0 (SPSS Inc, Chicago, Illinois). We conducted an assessment of the statistical power for a log-rank test comparing the 2 study groups given the available sample size. Setting an α level of .05 (2-sided), a sample size of 322 subjects, split approximately evenly between the 2 groups, yielded at least 80% statistical power when the hazard ratio for events among patients enrolled in a clinical trial relative to those not enrolled in any trial was 0.49 or lower. Assuming the overall EFS at the end of follow-up was 78% led to a detectable differential of 13.4% (84.9% survival among patients enrolled in a trial vs 71.5% among patients not enrolled in a clinical trial). This retrospective medical record review study was approved by the SCH institutional review board.