We investigated the influence of plasma concentrations of immunoglobulin G (IgG), IgA, IgM, IgG subclasses and mannan-binding lectin (MBL) on susceptibility to infection in patients with chronic lymphocytic leukemia (CLL). Of 28 patients with CLL, increased susceptibility to infection was recorded in nine. Four of them (44%) had hypogammaglobulinemia as opposed to only one (5%) of the 19 patients without increased susceptibility to infection (OR 14.4; 95% CI, 1.6-130). When the effect of IgG subclasses contributing to hypogammaglobulinemia was studied, only the decreased concentrations of IgG4 and IgG2 were associated with increased susceptibility to infection. They, in turn, were intercorrelated and also highly correlated with the concentration of IgA. In fact, when these parameters were studied by a multivariable model, only the decreased concentration of IgA was shown as an independent risk factor for infection (P = 0.03). The mean concentration of MBL was significantly higher in patients with infections than in those without (6.54 mg/l and 2.75 mg/l, respectively; P = 0.001). The monitoring of its concentrations might be useful in the follow-up of infectious morbidity in CLL.