In the early 1980s, volatile organic compounds (VOCs) were discovered in some of the water systems serving housing areas on Marine Corps Base Camp Lejeune. Exposure to certain VOCs may cause adverse health effects, including cancer. In 1999, the Department of Health and Human Services' (HHS) Agency for Toxic Substances and Disease Registry (ATSDR) began a study to examine whether individuals who were exposed in utero to the contaminated drinking water are more likely to have developed certain childhood cancers or birth defects. ATSDR has projected a December 2007 completion date for the study. The National Defense Authorization Act of Fiscal Year 2005 required GAO to report on past drinking water contamination and related health effects at Camp Lejeune. In this report GAO describes (1) efforts to identify and address the past contamination, (2) activities resulting from concerns about possible adverse health effects and government actions related to the past contamination, and (3) the design of the current ATSDR study, including the study's population, time frame, selected health effects, and the reasonableness of the projected completion date. GAO reviewed documents, interviewed officials and former residents, and contracted with the National Academy of Sciences to convene an expert panel to assess the design of the current ATSDR study.