Smoking cigarettes has been consistently associated with adenomatous polyps. However, only a few studies have reported associations between smoking cigarettes or using other forms of tobacco and colon cancer. A population-based case-control study of colon cancer was conducted in 3 areas in the United States: northern California, Utah and Minnesota. We observed approximately a 50% increase in colon cancer risk from smoking over a pack of cigarettes per day among both men and women. Those who stopped smoking remained at increased risk, even if they stopped over 10 years ago. Our data suggest that the amount smoked may be a more important factor than the total number of years smoked. Smoking neither cigars nor pipes was associated with an increased risk of colon cancer. Among female participants only, those who smoked over 20 cigarettes per day and had a large body mass index were at greater risk of colon cancer than participants who smoked the same amount but were smaller (p for interaction among women = 0.04).