This publication originates from a workshop held in 1997 to consider how rodent tumors of urinary bladder, renal cortex, and thyroid gland should be treated. The workshop also addressed the predictive value of these tumors for the identification of carcinogenic hazards to humans when they occur alone and when they occur along with tumors in other organs. In the following sections etiological risk factors in humans and experimental animals as well as the hypotheses underlying the proposed species-specific mechanisms for each of the above-mentioned tumor types are summarized. Finally, for each tumor type, recommendations are presented on how the mechanistic data could be used in the overall evaluation of carcinogenicity to humans. The Consensus report is published in the first part of this volume. Individual authored papers form the basis for discussions and are included in the second part of this volume.