A long-term follow-up of approximately 50,000 children who received the Hemophilus influenzae type b capsular polysaccharide vaccine in 1974 at three months to five years of age has shown good protective efficacy in those who received the vaccine at 18 months or older. No adverse effects were observed. Analysis of paired serum samples from 514 vaccinated children showed that effective immunization with this vaccine could be performed after but not before the age of 16 to 20 months. An analysis of 956 bacteremic H. influenzae infections occurring in Finland over a period of five years showed that 94 per cent of all cases were in children under 10 years of age. Of these, 40 per cent occurred in children under 18 months, and 60 per cent in children between the ages of 1 1/2 and 9 years. These 60 per cent are potentially preventable with the capsular polysaccharide vaccine.