The Girls health Enrichment Multi-site Studies (GEMS) is an obesity prevention research program sponsored by the National Heart, Lung, and Blood Institute (NHLBI), targeting young African-American girls. Expert groups have suggested that the high prevalence of obesity in African-American women could be a contributing factor to their excess morbidity and mortality from cardiovascular disease compared to women from other ethnic groups. To address the issue of obesity and its origins in African-American women, the NHLBI Growth and Health Study (NGHS) was initiated to investigate factors related to the development of obesity and associated cardiovascular disease risk factors in a cohort of young African-American and White girls, aged 9 and 10 years. Findings from NGHS, and the realization that obesity had become a major public health problem, subsequently led to a 2-phase, 7-year collaborative obesity prevention research program, the Girls health Enrichment Multi-site Studies (GEMS). Initiated in 1999, Phase 1 of GEMS was conducted collaboratively among 4 participating field centers, a coordinating center, and the NHLBI project office to conduct formative assessment research and to pilot test, over a 12-week period, interventions that might be effective in reducing the rate of weight gain in African-American girls, aged 8-10 years. Over a 2-year period, Phase 2 of GEMS will test the interventions that appear most promising in preventing excessive weight gain in young African-American girls. The experiences of the GEMS pilot studies will help guide future intervention research for obesity prevention beginning in childhood. This report describes the background and rationale for the GEMS initiative. This journal supplement describes the experiences of the GEMS Phase 1 program.