A prospective cohort study was designed to study the effects of family functioning and stress on the incidence of influenza infection. Families from the clinic roster, containing two adults and at least one child between the ages of 1 and 18 years, were asked to participate. Baseline (pre-influenza) data included a serum determination for influenza A and B antibodies, family functioning as measured by the Family Adaptability and Cohesion Evaluation Scales (FACES) II and the Family APGAR, and parental stress as measured by the social readjustment rating scale (SRRS). During the study all family members of patients with upper respiratory tract infection symptoms or fever were seen, and throat swabs were obtained for viral culture. Approximately 2 weeks after the influenza epidemic ended (March 1984), sera for antibodies were again collected on all family members. Chi-square analysis showed that infection (defined as a fourfold titer rise or a positive viral throat culture) was significantly associated with both cohesion and adaptability as measured by FACES II. Neither the Family APGAR nor the SRRS was associated with influenza B infection. It was concluded that family functioning affects the frequency of influenza B infection within families. This finding raises the possibility that family dysfunction may lead to altered immune responses, which increases susceptibility to infection.