Cigarette smoke has been implicated in a causal relationship with carcinoma of the lung. An intriguing feature of the disease is the site-selectivity with which bronchogenic cancer manifests itself; most cancers are detected in the main, lobar and segmental bronchi, perhaps specifically at airway bifurcations. The elevated risk of lung cancer to smokers may result from a complex interplay between smoking and exposure to ambient Rn progeny, including the promotional-effect role (as opposed to being the initiating event) of cigarette smoke in tumor development. It has been determined that smokers exposed to average indoor Rn progency levels receive surprisingly high a doses at 'hot spots' within bronchial bifurcations. (Copyright (c) 1987 Health Physics Society Pergamon Journals Ltd.)