Nowadays tumours represent one of the major problems of public health, both for their epidemiological dimension as for the causes that are at their origin, represented by three important factors: people ageing, general and occupational environmental pollution, including incongruous individual behaviour, and individual genetic susceptibility. The strategies followed till today in order to check tumours have privileged the role of therapeutic interventions: the results achieved show the great limits. On the contrary, current scientific experiences indicate the need to re-orientate present strategies, developing prevention programmes for the identification of carcinogenic risks (primary prevention), and early diagnosis of preneoplastic and neoplastic lesions (secondary prevention). In this review we discuss the importance of primary prevention in tumour control and specifically we present: - the role of long-term carcinogenic bioassays on rodents (rats and mice) in order to identify and predict the carcinogenic risks and then the great value that these studies have for public health; - the two major programmes of carcinogenic bioassays: the National Toxicological Program (NTP) of the US government, and the one performed by the European Ramazzini Foundation (ERF), in Italy; - the importance of observing the experimental animals until spontaneous death in order to improve the sensitivity and specificity of the assays, a way of working followed by ERF from the beginning.