Building mechanical systems play a major role in both initial cost and life cycle energy use in buildings. This research examines the role of design-build mechanical contractors in meeting energy efficiency and first cost objectives on high performance "green" building projects. Specifically, synergies between lean principles which eliminate process and materials waste are evaluated for alignment with green goals in resource minimization and energy efficiency. The role of design-build mechanical contractors in achieving green buildings is explored through engineering, design detailing, fabrication, and construction processes. Three illustrative case studies are summarized to illustrate the unique abilities of design-build mechanical contractors as valuable contributors on green projects. The results of these case studies and interviews with design-build mechanical professionals are summarized in descriptive tables in which waste reducing lean principles are applied to achieve green results. Key results include the value of integrated design and detailing of mechanical systems, the ability of design-build mechanical contractors to take on more risk with innovative design solutions, and the benefits of early involvement of expertise in estimating and constructability of mechanical systems on building projects. Implications for the design process of green buildings and the role of energy and mechanical engineering are discussed.