Type-specific immunoglobulin G (IgG) to pneumococcal capsular polysaccharide (CPS) and opsonic activity against Streptococcus pneumoniae were evaluated in serum samples from 36 Ugandan adults with community-acquired pneumonia and 58 asymptomatic Ugandan adults with or without human immunodeficiency virus type 1 (HIV-1) infection. The levels of serum IgG to CPS were significantly higher in HIV-1-infected subjects than in HIV-uninfected subjects. Serum samples from HIV-1-infected subjects that had lower IgG titers demonstrated higher opsonic activity against type 3 (titers of 7) and type 9 (titers of 7-11) pneumococcal strains. Plasma HIV-1 load also correlated inversely with serum opsonic activity against these strains, and peripheral blood CD4+ lymphocyte numbers also tended to correlate with serum opsonic activity in asymptomatic HIV-1-infected adults. Our findings suggest that the opsonic activity of type-specific IgG is impaired in the serum of HIV-1-infected African adults, which may expose them to a serious risk of invasive pneumococcal infections.