The dramatic advances made recently in human genome research are fueling considerable interest in genetic testing. A specific feature of genetic testing is that the results are final and impact not only the patient, but also the entire family. Those elements should be factored into the risk/benefit ratio evaluation. In France, legislation the differentiates diagnostic tests in symptomatic patients from predictive tests in asymptomatic patients with affected family members. Only multidisciplinary groups with both clinical and genetic expertise can order predictive tests. In all cases the physician must sign a statement that informed consent was obtained from the patient prior to testing. The test must be done in an accredited laboratory and the result communicated by the physician to the patient. Patient confidentiality must be respected, particularly regarding family members. In monogenic diseases, the diagnostic weight of genetic testing is often considerable, although the limitations should be borne in mind. In multifactorial diseases, genetic testing seeks to identify risk factors that are usually associated with a low level of risk. The result should be interpreted in the light of the clinical presentation, family history, and genetic background. The predictive value is closely dependent on the clinical presentation but is generally limited, most notably when family members are affected. Great care should be taken to avoid causing undue anxiety among individuals with positive test results. Diseases that illustrate these aspects include Paget's disease of bone, of which inherited variants caused by a mutation in a single gene on chromosome 5 have been identified recently, and rheumatoid arthritis, which is a multifactorial disease.