Patients with acute lymphoblastic leukemia who have a mediastinal mass at the time of diagnosis are said to have a poor prognosis. However, many factors which may not be independent contribute to the success of treatment. We compared the disease characteristics and results of therapy in children having large mediastinal masses and lymphoblastic leukemia without mediastinal mass. Patients with a mediastinal mass had less evidence of marrow failure but were burdened with considerably more leukemic cells as measured by peripheral blood white count and extent of lymphadenopathy. Since the presence of mediastinal mass was strongly associated with these and other poor prognostic characteristics, we used multivariate techniques to determine which characteristics were independently associated with an increased risk for relapse. White count, extent of lymphadenopathy, age, and sex were significant predictors of early relapse, but when controlled for these variables the presence of a mass did not predict prognosis.