To determine the clinical efficacy of combined vaccination with 23-valent pneumococcal vaccine (PV) and influenza vaccine (IV) against pneumonia and acute exacerbation of chronic lung diseases (CLD), we conducted an open-label, randomized, controlled study among 167 adults with CLD over a 2-year period. Subjects were randomly assigned to a PV+IV group (n=87) or an IV group (n=80). The number of patients with CLD experiencing infectious acute exacerbation (P=0.022), but not pneumonia (P=0.284), was significantly lower in the PV+IV group compared with the IV group. When these subjects were divided into subgroups, an additive effect of PV with IV in preventing infectious acute exacerbation was significant only in patients with chronic obstructive pulmonary diseases (P=0.037). In patients with CLD, the Kaplan-Meier survival curves demonstrated a significant difference for infectious acute exacerbation (P=0.016) between the two groups. An additive effect of PV with IV on infectious acute exacerbation was found during the first year after vaccination (P=0.019), but not during the second year (P=0.342), and was associated with serotype-specific immune response in sera of these patients who used PV during the same period.