BACKGROUND The impact of overt testicular disease on survival was assessed among patients treated at the study institution for childhood acute lymphoblastic leukemia (ALL) over a 15-year period. To the authors' knowledge, the frequency of overt testicular involvement at diagnosis of ALL and its impact on treatment outcome have not been reported previously. METHODS The medical records of all 651 boys with ALL enrolled on St. Jude Total Therapy Studies X-XIIIA (May 1979 to December 1993) were reviewed to determine the frequency of overt testicular involvement at diagnosis. The log rank test and sequential Cox regression analyses, adjusted for known adverse features, were used to compare event free and overall survival for patients with and without testicular leukemia. A matched-pairs analysis was then conducted for both outcome measures. RESULTS Thirteen of the 651 male patients (1.9%) presented with overt testicular leukemia. Compared with the other patients, these 13 boys had a significantly higher frequency of infant or adolescent age at diagnosis, hyperleukocytosis, splenomegaly, and mediastinal mass; a poorer 5-year event free survival (38% [95% confidence interval (CI), 15.4-61.4%] vs. 58% [95% CI, 53.6-61.7%], P = 0.06, log rank test); and a significantly poorer 5-year overall survival (37% [95% CI, 14-60.4%] vs. 75% [95% CI, 71.3-78.4%], P = 0.002). The difference in overall, but not event free, survival was supported by sequential Cox regression analyses and by the matched-pairs analysis (P = 0.04). CONCLUSIONS Overt testicular involvement with leukemia at diagnosis was associated with an adverse survival in boys with ALL. Testicular irradiation may not be necessary in those patients who present with overt testicular involvement. This finding merits prospective evaluation in a larger sample of similarly treated patients.