Molecular pathways and mechanisms underlying human cancer are frequently investigated in animal models. Studies to identify and dissect molecular pathways include the discovery of genes and their subsequent analysis in transgenic and 'knock-out' mice. A critical aspect in such investigations is the evaluation of organ integrity and histology upon the alteration or inactivation of specific genes. Results from such studies are usually published in scientific journals. However, due to print space and costs the display of large high quality images is limited. Furthermore, the printed media does not permit an easy comparison of histological images published in different journals and different years. The Internet provides a tool for the timely and inexpensive dissemination of scientific data to the research community. However, its potential for the analysis of histological images has not been explored. Here we present a web-based interactive histology atlas (http://histology.nih.gov) that permits the retrieval of annotated, high-resolution histology images via the Internet. This histology atlas also takes advantage of the interactive nature of the Internet to support the communication between different research groups. As an outline forum this atlas provides the framework to evaluate and understand cancer pathology, and to develop a consensus between veterinary and human pathologists.