A screening for plasma cholesterol levels was conducted at six sites in the New York metropolitan area and involved hospitals, health professionals, paraprofessionals, media experts, and instruments that provided cholesterol levels rapidly. During the five days of the testing, over 12 000 participants were screened. Because the program was limited to customary working hours and because of self-selection of participants, the subjects were probably an unusually health-conscious group as evidenced by the low prevalence of cigarette smokers (11%). Nevertheless, 12% were at moderate risk and 16% were at high risk for coronary heart disease. Approximately half of the population reported never having had their cholesterol levels tested, and over 40% had no idea what levels were optimal. A subsample of patients at risk was screened by telephone survey. In the majority of cases, when a patient's physician was consulted for advice, no action was recommended. Our results demonstrate that a large population screening can be implemented, that at least certain segments of the public will respond to such a program, and that educational efforts must be directed at both the public at large and physicians.