Patients with cancers caused by smoking may die because they continue to smoke even after diagnosis of a cancer caused by smoking. We investigated differences in cause-specific mortality between patients diagnosed with smoking-related and non-smoking-related cancers. The causes of death were classified as smoking-related cancer, non-smoking-related cancer, respiratory or vascular disease, and all other causes. We studied all 220 089 people diagnosed with cancer in Scotland between 1986 and 1996, aged between 20 and 85 years, with last follow-up on 31 December 1999. There was a moderate excess risk of dying from respiratory and vascular causes in those with smoking-related cancers, which did not fall with time since diagnosis, consistent with continued smoking by these patients. Mortality among cancer patients might fall if more assistance in stopping smoking was provided for patients who have smoking-related cancers.