Previous studies have reported total leukocyte count to be a risk factor for coronary heart disease. The pattern of association between each class of leukocytes and the onset of the disease was investigated in the Paris Prospective Study II. The study population consisted of 3,779 male French employees aged 29-52 years working in the Paris area and recruited between 1980 and 1985 (end of follow-up: June 1989). A high monocyte count was strongly associated with the risk of coronary heart disease after adjustment for tobacco use and the other classical risk factors for the disease. The monocyte-macrophage is known to play an important part in the development of atherosclerosis. A high monocyte count seems to predict the premature occurrence of a coronary event in middle-aged men.