Important contributions to the success of this study were made by the following: Department of Veterans Affairs Health Services and Development Service: Chief Research and Development Officer, John R. Feussner, MD; Former Director, Daniel Deykin, MD; Administrative Officer, Janet Gold; Cooperative Studies in Health Services: Program Manager, Charles Welch III, PhD; Health Services Research & Development: Deputy Director, Shirley Meehan, MBA, PhD; Hines Veterans Administration Cooperative Studies Program Coordinating Center, Vietnam Era Twin Registry: Director, William G. Henderson, PhD; Registry Coordinator, Mary Ellen Vitek; Programmer, Kenneth Bukowski; Statistical Assistant, Mary Biondic; Vietnam Era Twin Registry Advisory Committee: Theodore Colton, ScD, Ralph Paffenbarger, MD, Walter Nance, MD, Myrna Weissman, PhD, and Roger Williams, MD. Irving Gottesman, PhD, and Jag Khalsa, PhD, also made important contributions. Most important, we thank the members of the Vietnam Era Twin Registry for their participation and cooperation. They willingly provided sensitive information and considerable time in responding to the survey. Without their contribution this research project would not have been possible.