Long-term animal carcinogenicity studies are an important part of the risk analysis process assessing the carcinogenic potential of products to humans. Results from the statistical analysis of the data from such studies are generally presented as a series of hypothesis tests indicating whether there was a significant rise in the number of tumors at given sites. The conclusion from such an analysis depends on the size of the experiment. In particular, the number of false-negative results can be high when tumors are rare. In this paper, a test for equivalence fixing the proportion of false negatives is proposed. The effect on the required sample size is also discussed.