This article examines the role of biologic and environmental factors in det
ermining the long-term outcomes of extremely low-birth weight infants. Rese
arch focusing on follow-up to at least 4 years of age is reviewed. Methodol
ogic issues related to sampling, the use of control groups, and diagnostic
criteria are also discussed. The use of cumulative models of risk for exami
ning the relative contribution of environmental and biologic factors is pre
sented.