Subjects within a family have a high probability of sharing environmental factors as well as genes. Without any previous hypothesis, familial aggregation cannot be attributed to genetic or environmental factors. In epidemiology a common measure of familial aggregation is the odds ratio (OR) evaluated from case-control studies which gives a measure of the increased risk associated with particular aspects of the family's history of disease. An analytical method is presented to model the OR of case-control studies in the presence of risk factors, genetic or environmental, that are correlated between relatives. A graphical approach that can be used to link the parameters of the model with the values of the OR is also described. The method is illustrated using the example of familial aggregation in colorectal cancer.