We measured the serum concentrations of IgG subclasses in healthy subjects (n = 26) and in patients with community-acquired pneumonia (CAP) on admission (n = 38), at recovery (n = 21), and 9 months after admission (n = 19). Then, in 8 of the control subjects and 15 of the patients, we measured IgG subclasses and mean serum antibody concentrations of pneumococcal capsular polysaccharides before and 3 wk after immunization with a pneumococcal vaccine. Compared to the control subjects, the serum concentration of the IgG2 subclass was lower at admission in patients with CAP of bacterial or unknown cause (p less than 0.005). Concentrations of IgG subclasses in patients did not differ between admission and recovery, or between admission and 9 months later. After vaccination, in both control subjects and patients, there was an increase in the concentrations of IgG2 subclasses (p = 0.01) and antipneumococcal antibodies (p less than 10(-4)). We show that serum IgG2 concentration in patients with CAP of bacterial or unknown cause is lower than in healthy subjects and remains lower for several months. After immunization with a pneumococcal vaccine, the increase in serum concentrations of IgG subclasses and antipneumococcal antibodies in patients does not differ from those in control subjects.