Today man is more than ever exposed to the danger of cancer; especially the increase of the cancer of the lungs is terrifying. By demonstrating the causes of cancer, this lecture is noving the thesis, that in most cases cancer is acquired by means of exogen causes. Cancer increases progressivily, because the causes increase, on which it depends. The factor "owing to old age" is interpreted in the way, that old age by itself does not cause cancer, but that it gives many carcinogenic substances the possibility to augment and cumulate in continued dayly doses, till at last the endsum of carcino- genic material achieves the final transformation to cancer. Old age makes cancer possible, but it is not its cause. The cancer-therapy has now a result of 55 °/o five-years-survival for state I, for all states together only 35 °/o. That means at the same time, that inspite of all helping therapy 65 °/o of all cancer patients stay uncured, at least in the long run. As we cannot count on a drastic improvement of our curing results, according to our knowledge of today, we must suppose, that cancer will still increase, when the old cancer-noxae persist and new ones are added. By extrapolating the curves of today, we must realise, that soon after the year 2000 every 4th man will succumb to cancer. So the cancer-prophylaxis has an always increasing importance.