An adult child's provision of care to an unmarried elderly mother varies both within and between families. Within-family differences address the variation in different children's behavior within in a family. Between-family differences refer to the propensities that members of a family--the children of one mother--share and that differentiate them from other families. Previous research suggests 5 hypotheses affecting either within-family or between-family differences. Data from multiple waves of the Asset and Health Dynamics Among the Oldest Old (AHEAD) cohort of the Health and Retirement Study (HRS; 16,719 observations on 5,607 mother-child dyads in 1,925 families) are used to estimate a multilevel model with a binary outcome. Results indicate substantial differences between families. Mother's characteristics, family composition, and family history account for about half the between-family differences.