A prospective, population-based study of the incidence of infections with Haemophilus influenzae in Fresno County, California, was conducted. The average annual incidence or meningitis was 5.18 cases per 100,000 population, and of other invasive infections, 3.48 cases per 100,000. Incidence rates were highest in infants, but 13% of infections and five of seven deaths were in adults, primarily in elderly individuals with compromised defenses. Black children four years of age and younger were more susceptible to meningitis due to H. influenzae than were white children (P less than 0.001). In contrast, the increased incidence of haemophilus disease in Hispanic children was primarily a result of invasive infections other than meningitis (P less than 0.03). The secondary attack rate in household contacts less than two years of age was one in 24 during the first 30 days, one in 23 between 31 and 60 days, and 0.25 in 22 per month between three and six months. Age and racial factors may affect susceptibility to different forms of invasive disease due to H. influenzae, and young contacts of a patient are at increased risk of developing illness.