In 1930, a German physician wrote a paper in which he most strongly suggested that smoking of cigarettes is a cause of lung cancer (1) . In the same year, Mertens (2) published the results of a study in which he had exposed individual mice to cigarette smoke. He used a compressible rubber bulb to force cigarette smoke into a glass desiccator. Exposures were from 1-4 h daily, for up to 15 mo. Upon histological examination, he observed inflammatory changes in the lungs, although he was quick to point out that large areas of the lungs showed no pathological alterations. Neoplastic lung lesions were found in two animals. In one mouse, multiple small nodules, classified as adenocarcinomas, were considered to have been pre-existing. The second animal showed several small nodules and, in addition, one large adenocarcinoma, mm in diameter, originating in a bronchus and invading the adjacent parenchyma. The author summarized his findings by stating that he had found one bronchial cancer, but that it was doubtful whether it had been caused by tobacco smoke.