In family-based treatment (FBT) for adolescent anorexia nervosa, all family members are encouraged to attend sessions with the understanding that absences negatively impact treatment. There are, however, many obstacles to family members' attendance, and there is no research to indicate whether family member attendance improves treatment outcomes. We examined attendance patterns of 198 families who participated in FBT at a specialist pediatric eating disorders program and assessed the extent to which participation by mothers (n = 194), fathers (n = 175), and siblings (n = 165; 50% female) predicted outcome. All mothers attended at least one session, and 74% attended all sessions. By comparison, 95% of fathers and 73% of siblings attended at least one session, and 33% of fathers and 1% of siblings attended all sessions. The mean proportion of sessions attended was 94% for mothers, 72% for fathers, and 20% for siblings. Over 6 months of treatment, the proportion of mothers who attended each session was largely stable; fathers' attendance declined slowly, and siblings' attendance declined more rapidly. Greater attendance by fathers predicted higher weight and lower eating disorder symptoms in adolescents at end of treatment. Remission at end of treatment was associated with higher attendance by fathers (M = 81% vs. M = 69%). Achieving sustained engagement of the whole family system in FBT is a considerable challenge. However, this study demonstrates that implementing processes that encourage and enable family members to attend treatment sessions could have significant benefits for patient outcomes.