Complement deficiencies are known to be associated with increased susceptibility to bacterial infections. In the present study we investigated 80 patients with either a history of pneumococcal bacteremic infection, or recurrent pneumonia, or both. Hemolytic screening tests for complement deficiency were performed and serum concentrations of C1q, C1s, C2, C3, C4, C4 isotypes, factor B, factor D, and properdin were determined. Complete deficiencies of single complement proteins were not found. 10 patients (12%) had a C4 isotype deficiency, but the frequency of homozygous C4A and C4B deficiency was not significantly increased. Seven patients (9%) had hypocomplementemia with low concentrations of at least 2 complement proteins. One of these patients had profound depletion of classical pathway components and findings suggesting acquired C1 esterase inhibitor deficiency. 16 patients (20%) had minor complement aberrations. A majority of the patients with hypocomplementemia suffered from other conditions associated with pneumococcal infections. However, impaired complement function could be a significant predisposing factor in some patients with invasive pneumococcal infections or recurrent pneumonia.