Familial risk ratios for high blood pressure were estimated in a representative sample of the Canadian population. The sample consisted of 14,069 participants 7-69 years of age from 5,753 families participating in the 1981 Canada Fitness Survey. Resting systolic (SBP) and diastolic (DBP) blood pressures were adjusted for the effects of body mass index using regression procedures. Varying degrees of high blood pressure were defined as the 75(th), 85(th), and 95(th) percentiles of age- and sex-specific values. Age- and sex-standardized risk ratios (SRRs) were calculated comparing the prevalences in the general population to those in spouses and first-degree relatives of probands with high blood pressure. SRRs for the 95(th) percentile were, for SBP and DBP, respectively, 1.37 and 1.45 in spouses and 1.33 and 2.36 in first-degree relatives of probands. SRRs decrease with decreasing percentile cut-offs used to define high blood pressure (95(th) > 85(th) > 75(th)), and SRRs are generally higher in first-degree relatives than in spouses, particularly for DBP. The results indicate significant familial risk for high blood pressure in the Canadian population, and the pattern of SRRs suggests that genetic factors may be responsible for a portion of the risk.