[Importance of the definition of urban zones in the study of cancer risk factors].

1446 cases of colorectal cancer have been collected in the Registry of digestive tract tumours in the "Department" of Calvados with the aim of studying the effect of residential location on cancer incidence. The risk-ratio was different in males and in females and between different types of urban areas. Using the urban category of "Zone de Peuplement Industriel ou Urbain" (ZPIU) increased the observed risk-ratio (1.6 for males, 1.2 for females), and enabled definition of homogeneous populations. This improved classification, thus allowed a more discriminating analysis of the effect of residential location on risk of cancer in both sexes.