The development, implementation, and results of the smoking cessation program of the Multiple Risk Factor Intervention Trial (MRFIT) are presented. The MRFIT is a 6-year clinical trial designed to investigate the effects of reducing cardiovascular risk factors -- elevated cholesterol, hypertension, and cigarette smoking -- in a group of asymptomatic men at high risk of cardiovascular disease. The men participated in an integrated intervention program that offered both group and individual formats, a structured maintenance program for those who stopped smoking, and an extended intervention program for those unable to quit initially. Results among the original 4,103 smokers included a 47.3% quit rate 4 months after program initiation and a 45.9% quit rate after 4 years. Of those reporting no smoking at 4 months, 56% were abstinent at all visits through 48 months. Most recidivism occurred soon after initial cessation, with 17% of the men who reported quitting at 4 months reporting smoking 4 months later. The quit rates were strongly associated with the initial level of smoking, with light smokers reporting higher quit rates and lower recidivism rates at all visits through 4 years. Results exceed trial goals whether measured by self-reports or by thiocyanate levels, an objective assessment of smoking behavior. Discussion focuses on understanding the variables contributing to smoking cessation and to achieving the goals of reduction of risk of cardiovascular disease.