Risk factors for cancer have been found in the past to act synergistically in a number of studies. However, these studies were not always designed to test the hypothesis of synergism, and have sometimes failed to equate for important variables, which might influence the results. The present study tests the hypothesis that psychosocial variables and physical ones (personality/stress, smoking, and genetic predisposition) interact in a synergistic fashion in the causation of lung cancer and coronary heart disease (CHD).