We examined the extent to which lay people and health professionals are able to assess occurrence risks for multifactorial diseases. We asked 341 participants to assess the risk of developing lung cancer, coronary artery disease or rheumatoid arthritis in 16 scenarios, each featuring a combination of four factors (family history, daily alcohol intake, daily tobacco consumption and genetic test results). Participants considered all factors. However, they accorded more weight to tobacco and genetic test results. Moreover, it appears that where one of the factors (e.g. the presence of the incriminated gene) exerted a strong influence, the influence of the other factor(s) was correspondingly weaker. The health risk judgements of health professionals were more dependent on the specific disease and were also influenced to a greater degree by genetic information than lay people.