Few prospective studies of child growth and its determinants take place in programmatic contexts. We evaluated the effect of Save the Children's (SC) community empowerment and nutrition program (CENP) on child growth, care, morbidity, empowerment, and behavioral determinants. This paper describes the research methods of this community-based study. We used a longitudinal, prospective, randomized design. We selected 12 impoverished communes with documented child malnutrition, three comparison, and three intervention communes in each of two districts in Phu Tho Province, west of Hanoi. SC taught district trainers in November 1999 to train local health volunteers to implement the 10-month CENP, including situation analysis, positive deviance (PD) inquiry, growth monitoring and promotion, nutrition education and rehabilitation program (NERP), deworming, and monitoring. PD inquiries aim to discover successful care practices in poor households that likely promote well-nourished children. NERPs are neighborhood-based, facilitated group learning sessions where caregivers of malnourished children learn and practice PD and other healthy behaviors. We dewormed all intervention and comparison children. We randomly selected 240 children 5 to 25 months of age (120 intervention and 120 comparison). We gathered information on nutritional status, diet, illness, care, behavioral determinants, empowerment, and program quality, monthly for six months with a re-survey at 12 months. We collected most information through maternal interview but also observed hygiene and program quality, and videotaped feedings at home. Some implementation and research limitations will attenuate CENP impact and measurement of its effectiveness.