This study examined the association between ecological context (extrafamilial, familial, child factors) at baseline and longitudinal retention of families in the 36-month assessment of an adolescent alcohol and tobacco use prevention program that was conducted within a pediatric primary care setting. A total of 1,780 families were enrolled at baseline when the youth were in the fifth and sixth grades, and 1,220 of these families participated in the 36-month assessment. Findings indicated that familial and child, but not extrafamilial, factors were associated with the participation of families in the 36-month assessment. Clinical implications and future research directions are discussed. (Contains 2 tables.)