The purpose of this study was to determine the extent to which members of families have similar and interrelated health behavior. Utilization and morbidity data from more than 80,000 ambulatory visits by 693 families enrolled in a prepaid health plan for 6 consecutive years were examined using family membership as the major unit of analysis. Family members were found to resemble one another in their rates of use of services (r = 0.44; P less than 0.001). Parental influence on children's utilization is significant, mothers generally being two to three times more powerful than fathers in this regard. Apart from family size, structural characteristics of families contribute little to family health behavior. Family membership explains nearly one third of the variance of individual utilization even after family size and age and sex of family members are considered. A small proportion of families (5%) accounts for a disproportionately large share (12.3%) of health care utilization. Intrafamilial patterns of morbidity were apparent for several major groupings of diagnoses, most notably for acute health problems. Intrafamilial correlations were greatest among high-utilizing families. Families establish patterns of health behavior that are stable over time and therefore may be amenable to selective interventions. We conclude that health care planning, whether for service delivery or health education intervention, should consider family health data as an important information source.