A total of 4,604 men who were interviewed in Finland in 1962 in connection with the Finnish-Norwegian lung-cancer study were followed-up for lung cancer during 1963-87 to establish why urbanized (via migration) men who smoked had a greater lung-cancer risk than native urban smokers. Exposure to occupational carcinogens was inferred from the title of the longest job held. A clear dose-response relation between occupational exposure and lung cancer was found in the urbanized but not among the native urban dwellers. The extra risk associated with migration to towns and smoking was found especially by those urbanized subjects who worked in heavily exposed industries: their lung cancer risk was more than twice that of native urban men in similar jobs, while those urbanized subjects in academic or clerical jobs showed no increased risk when compared with native urban men in corresponding work. Cardiorespiratory symptoms had a prognostic value in every residential group, but especially among the urbanized. Urbanized men who smoked and worked in heavily exposed industries, and suffered from shortness of breath, had a fourfold risk of lung cancer when compared with native urban smokers without this symptom. We conclude that although the joint effect of smoking and occupational exposure is the main explanatory factor for high risk of lung cancer in urbanized males, environmental and psychosocial factors also may have a contributory effect.