Survival in 73 patients with Ph1-positive chronic myelocytic leukemia was correlated with remission duration and leukocyte doubling time after initial treatment with busulfan and also, with clinical and laboratory features recorded at the time of diagnosis. There was a significant relationship between remission duration and leukocyte doubling time (correlation coefficient 0.94). On multivariate analysis, the most important factor influencing remission duration (and doubling time) was the percentage of circulating blasts (p less than 0.001). The spleen size (p less than 0.02), Liver size (p less than 0.03) and WBC (p less than 0.03) also added significantly to model fit; however, once a second variable was entered into the model no other factor produced significant improvement in model fit. In univariate analyses, the remission duration, doubling time, percentage of circulating blasts, spleen and liver size correlated significantly with survival. Multivariate analysis indicated that the percentage of circulating blasts was the most important factor affecting survival (p less than 0.001), with the liver size adding significantly to model fit (p less than 0.05). Remission duration, leukocyte doubling time and spleen size did not significantly influence survival once the percentage of blasts was included in the model. Thus, evaluation of clinical and laboratory data at the time of diagnosis is more important for prognostic classification of patients than the measurement of relapse kinetics after initial treatment with busulfan.