Community-academic partnerships have demonstrated potential for studying and improving community and environmental health, but only recently have their policy impacts been systematically studied. This case study highlights the evolution, research, and policy processes and outcomes of a community based participatory research (CBPR) partnership that has had multilevel impacts on health policy concerning diesel bus emissions and related environmental justice issues. The partnership between West Harlem Environmental ACTion, Inc. (WE ACT) and the Columbia University Center for Children's Environmental Health was explored using a multimethod case study approach. The conversion of New York City's bus fleet to clean diesel and the installation by the EPA of permanent air monitors in Harlem and other "hot spots" were among the outcomes for which the partnership's research and policy work was given substantial credit. Lessons for other urban community-academic partnerships interested in using CBPR to promote healthy public policy are discussed.