Subject loss in a cohort of 645 infants and their families, enrolled in a multicenter clinical trial, was described. Medical/biologic, socioeconomic and social support (Environmental Quality Index [EQI] composite), and developmental outcome data were obtained. Dropout was evaluated by comparing infants who withdrew at any time throughout the study to those who remained, as well as at four different time periods between 40 weeks conceptional age and 36 months. A dropout profile was also developed. Overall dropout was predicted by EQI score, clinical center, gestational age category, and 18-month developmental status. In addition to these variables, the dropout profile included race, but not 18-month developmental status. Medical/biologic variables, environmental quality, and race and center were most influential in the first, second and third, and fourth time periods, respectively. The identification of factors which are associated with increased subject loss can help researchers project needed sample sizes in future studies.