Some have proposed that 2-year carcinogenicity studies may not be necessary if the material is a direct-acting DNA mutagen, induces liver enzymes, causes hyperplasia or toxicity in particular organs, causes cell proliferation, is cytotoxic, causes hormonal perturbations, or if one has QSAR analyses or 'omics information. Safety pharmacology data, pharmacologic activity, metabolism data, and results of 13-week dose ranging studies (with organ weight data, clinical chemistry data, hematologic data, clinical signs and histopathologic findings) were compared with results of 2-year carcinogenicity studies reviewed by the Center for Drug Evaluation and Research (CDER)/FDA. The experience with the ICH genetic toxicology battery and alternative carcinogenicity models was also reviewed. It appears that the information available from short-term studies is not currently sufficient to accurately and reliably predict the outcome of long-term carcinogenicity studies.