In 1987, the Health Effects Institute entered into a partnership with the National Toxicology Program (NTP) to evaluate the effects of prolonged ozone exposure on F344/N rats. The NTP studies focused on carcinogenicity. HEI funded eight independent research studies, including investigations of lung biochemical constituents, structural and cellular changes, lung function, and nasal structure and function. In addition, a Biostatistical Advisory Group developed an animal allocation scheme that allowed several investigators to measure endpoints on the same set of animals, assisted the individual investigators with data analyses, and developed a statistical approach for analyzing multiple endpoints across the individual studies. The Integrative Summary of the Collaborative Ozone Project Group highlights the major findings, integrates the results, and discusses their implications for human health.