This summary briefly covers smoking as a primary risk factor for cancer, but the main focus is on the effect of smoking on recurrence or on diagnosis of a second primary cancer; patterns of quitting and continued smoking in cancer patients; and recommendations for smoking intervention for cancer patients. This information will assist health professionals caring for patients during and after treatment. Substantial material on cancer prevention and smoking cessation in the general population is available elsewhere. Because virtually all the available evidence pertains to cigarettes rather than other forms of tobacco such as snuff or chewing tobacco, reference is made to smoking cigarettes rather than to tobacco use.