The purpose of this study was to assess quantified measures of intrafamily patterns of influence on interdental cleaning behavior based on a conceptual model of family influences on health behavior. Data stemmed from the Norwegian National Health Survey 1985, comprising 295 two-parent families with 2 children above 6 years of age. Analyses were performed with the family as the sampling unit. Logistic regression analyses were applied. The results showed that there were statistically significant associations of interdental cleaning behavior among all pairs of family members, the strongest association being between sibling (OR = 36.6). The multivariate analyses of interdental cleaning behavior of the children showed that the strongest explanatory factor was the interdental cleaning behavior of the mother (OR = 2.4). Thus, the mother seems to play an important rôle in the formation and maintenance of the elder child's interdental cleaning behavior. The highly correlated interdental cleaning behavior of the 2 siblings could be explained by correlated measurement errors as well as equity in rules and parental control and reinforcement of the behavior, in addition to substantive influential effect of the elder sibling's interdental cleaning behavior on that of the younger sibling.