Children with ALL who have a high probability of excellent response, long-term survival, and cure when treated with widely available standard therapy can be identified at the time of diagnosis. Others can be identified who are at high risk of failure to respond, early relapse, and death. The importance of such prognostic characteristics is so great that they should be considered in the selection of appropriate treatment for each patient. Prognostic factors also must be considered in the design and analysis of clinical trials. All prognostic factors are not equally significant and many of them are closely associated. It is possible to rank them in importance and usefulness by appropriate statistical methods. As additional prognostic factors are identified, and as more effective and more specific therapies are developed, the relative importance of prognostic factors will change.