AbstractIn most diseases of contemporary interest to epidemiology, the importance of familial risk factors is often assessed by relative risk measures computed from family history data. In this paper, we examine the efficiency of relative risk measures in discerning the genetic background of a disease. It is shown that even for a simple (one locus, two alleles) genetic disease, incomplete penetrance of the disease allele and/or occurrence of sporadics (i.e. non‐heritable cases caused by environmental risk factors) often yields a rather small value of relative risk. This means that a small value of relative risk need not necessarily imply that genetic factors are unimportant in the aetiology of the disease. Furthermore, it is shown that the relative risk measures are complicated functions of disease gene frequency, the penetrance parameter and the sporadic rate. A rejection of genetic involvement for a disease on the basis of a single relative risk estimate may, therefore, be erroneou