Invasive Haemophilus influenzae type b (Hib) disease occurs with unusually high incidence in Alaskan Eskimos. In 1983, the authors evaluated the unique susceptibility of the Yupik-speaking Eskimo population in southwest Alaska. A matched case-control design was used to assess the influence of age, breast feeding, and household composition on disease risk, with a historical cohort design to evaluate their effects on acquisition of Hib anticapsular antibody. The authors studied 103 cases with known invasive Hib disease that occurred at a mean age of 8.7 +/- 8.6 months; healthy controls were matched for age and village of residence. Living in extended families at the time of disease onset was significantly associated with Hib disease (p less than 0.04; odds ratio = 1.8; 95% confidence interval 0.87-3.25). The authors found that breast feeding was significantly less common among cases than controls (p less than 0.03; odds ratio = 0.53; 95% confidence interval 0.27-0.98). Although there was a positive correlation between age and acquired level of total anticapsular antibody (r = 0.59; p less than 0.0001), previous exposure to invasive Hib disease did not influence these levels. Household crowding and breast feeding also did not appear to affect Hib antibody acquisition.