The use of different registration rules from one registry to another, both generally and also for paired organs, leads to variations in the proportion of multiple primary cancers: in men, from 0.4 to 4.9% for the colon, 0.1 to 2.7% for the lung, and 4.1 to 8.6% for the mouth and pharynx. Subjective factors, often impossible to verify, contribute to these variations. The impact on the estimation of incidence rates and trends is not negligible for cancers of the mouth and the pharynx and for all the cancers taken together. The trend towards an increase of cancers of the mouth and pharynx in the Bas-Rhin disappeared when the incidence was expressed taking only the first cancer (incidence by individual) into consideration, and the differences in incidence between the Calvados and the Bas-Rhin registries for the same site also disappeared. In the absence of harmonisation of the rules and methods followed for registration, incidence by individual is the only approach which makes it possible to compare incidence rates and trends between registries.