Information from examinations of 62,541 adults enrolled in a study of smoking from 1979 through 1982 in Oakland, California, was used to explore the associations of various measures of intensity of smoking with the leukocyte count and to try to determine whether there was a persistent effect of smoking cigarettes on the leukocyte count. In current, regular cigarette smokers, leukocyte counts were higher in smokers of a large number of cigarettes and were associated with smoking cigarettes with a high tar and nicotine yield, deep inhalation of the cigarette smoke, and a longer duration of smoking. There was an association of past smoking with a high leukocyte count independent of age, sex, and race. In past smokers of cigarettes who used no other form of tobacco, the leukocyte count was related to time since quitting, smokers who had quit more recently having higher leukocyte counts. The study shows that a high leukocyte count is associated consistently with various measures of intensity of cigarette smoking. Moreover, it appears that smoking has an effect on the leukocyte count that persists after quitting. Delineation of the physiologic basis for the acute and chronic effects of cigarette smoking on the leukocyte count might lead eventually to a better understanding of the mechanisms for regulation of granulopoiesis and the release and destruction of leukocytes.