A population-based matched case-control analysis of risk factors of invasive Haemophilus influenzae type b (Hib) disease was conducted in Finland in 1985 and 1986 before large-scale Hib vaccinations; 117 consecutive child patients with invasive Hib disease and 225 control subjects matched for age, sex, and residence were studied. In the multivariate analysis, day care outside the home was found to increase the risk of invasive Hib disease (odds ratio 5, 95% confidence interval 2.3 to 11), with the highest risk among children less than 2 years of age; this risk was significantly higher within the first month of attendance than later on (p = 0.02). The existence of siblings less than 7 years of age was found to be a risk factor, especially for the younger children (odds ratio 8.6, 95% confidence interval 2.6 to 52 for children less than 1 year of age), and the odds ratio increased approximately twofold with each additional sibling. A history of otitis media and previous hospitalizations were further risk factors for invasive Hib disease (odds ratio 2.2, 95% confidence interval 1.2 to 3.9, and odds ratio 1.9, 95% confidence interval 1.0 to 3.4, respectively). Breast-feeding for longer than 6 months was found to be protective (odds ratio 0.47, 95% confidence interval 0.3 to 0.9). The amount of Hib disease in different populations will vary with the incidence of these risk factors.