Low concentrations of mannose-binding protein (MBP; also known as mannose-binding lectin) are associated with common opsonic defect in immunodeficient children. We compared the concentrations of MBP in the sera of 47 adults with non-human immunodeficiency virus-related recurrent infections (group I) and 50 healthy adult controls. Mean serum MBP concentrations in the patient group did not differ significantly from those in the control group (P < 0.4). Nevertheless, the proportion of individuals with less than 5 ng of serum MBP per ml was significantly larger in the patient group (21%, P = 0.01) than in the control group (4%). Group II consisted of 73 pediatric and 56 adult patients with recurrent infections. Pediatric patients had significantly lower mean concentrations of serum MBP than their controls (P < 0.005), and there was no significant difference between the concentrations in sera of adult patients and adult controls (P < 0.4). Again, the proportion of individuals with less than 5 ng of serum MBP per ml was significantly larger in both pediatric (22%, P = 0.045) and adult (38%, P = 0.000016) patients than in their respective controls (4%). Our results demonstrate that, as in children, low concentrations of serum MBP can be associated with recurrent infections in adults.