To obtain information relevant to the question of bone marrow transplantation, we examined the prognostic significance of disease features recorded at the time of diagnosis among 625 patients, aged 5 to 45, with Philadelphia chromosome-positive, nonblastic chronic granulocytic leukemia. The actuarial death rate for this population was 5% during the first year after diagnosis, 12% during the second year, and averaged 22.5% per year during the next eight years. Multivariable regression analysis of features recorded in nearly all cases indicated that sex, spleen size, hematocrit, platelet count, and percentage of circulating blasts were significant prognostic indicators. Analyses of additional data available in 113 to 421 cases suggested that serum lactic dehydrogenase activity, percentage of blasts in marrow, nucleated RBCs in blood, and percentage of basophils plus eosinophils might also provide useful prognostic information. A Cox model, generated with five variables representing features recorded regularly (the first five listed), permitted segregation of these patients into three groups with significantly different survival patterns. The high- risk group exhibited an actuarial mortality of 30% during the first two years after diagnosis and an annual risk of 30% thereafter. In contrast, the most favorable group had a two-year actuarial mortality of 9% and an average risk thereafter of 17% per year, with a median survival of 5 1/2 years. We conclude that it should be possible to classify potential candidates for bone marrow transplantation according to risk with conventional therapy. Such information may be useful in making decisions regarding early v deferred marrow transplantation.