We reviewed the records of all patients with chronic myeloid leukemia (CML) seen in the CML Clinic of the University of Colorado Health Sciences Center between 1968 and 1987 for a history of cigarette smoking. Patients who smoked for five or more pack/years within the ten years preceding, or after the diagnosis of CML, were defined as smokers. Adequate smoking histories were obtained on 122 patients. Eighty-seven of these were non-smokers and 35 were smokers by the above criteria. The smoking group had a higher predominance of males, an older median age, and were diagnosed earlier in the course of the 20 year study. Seventy-two patients had died at the time of analysis. All but one, a non-smoker, died from the development of blast crisis. The overall median actuarial survival was significantly reduced for smokers (35 months) as compared to non-smokers (47 months). This was particularly striking for patients who had succumbed to the disease, with a median survival of 30 months in smokers versus 46 months in non-smokers. Although various explanations could explain the differences noted, we conclude that cigarette smoking has an adverse effect on the development of blast crisis and survival in chronic myeloid leukemia.