Cigarette smoke is by far the major extrinsic (non-genetic) cause of lung cancer. A small number of cases of this disease may be due to other environmental factors which act either independently or synergistically with tobacco smoke. The effect of environmental carcinogens is most obvious in occupational settings where the responsible substance may occur in relatively high concentrations. However, certain substances may cause carcinogenesis even after non-occupational exposure. The detailed study of environmental effects is fraught with methodological problems. However, it is clear that factors such as radon, arsenic, asbestos, urban pollution, and to a lesser degree socioeconomic conditions and nutrition, may be implicated in a small number of lung cancer cases. The avoidance of all these factors is not practical, and is not expected to have a significant impact on the incidence of lung cancer as long as the major carcinogen, tobacco, is not effectively controlled. Pneumon 2003, 16(1):29-37.