To gain insight into the possible role of mannose-binding lectin (MBL) in HIV infection, we analyzed serum levels and the functional complement activation capacity of MBL in different clinical stages of HIV infection during cross-sectional analysis (n = 62) and longitudinal testing (n = 23) as well as during highly active antiretroviral therapy (HAART) (n = 40). The results were correlated with serum levels of C-reactive protein (CRP). Our main findings were as follows. MBL levels and the capacity of complement activation by the MBL pathway were increased in HIV-infected patients with advanced clinical disease as shown in both cross-sectional analysis and longitudinal testing. There was no "normalization" of these parameters during HAART. In fact, MBL levels increased during therapy, and this increase was associated with a good virologic response. Although both MBL and CRP are regarded as acute-phase proteins, no correlation was seen between these proteins. Thus, the notably diverse patterns of MBL responses among patients with different clinical courses and treatments suggest that MBL and complement activation by the MBL pathway could be involved in the pathophysiology of HIV infection. It is not inconceivable that the net effects of MBL responses may vary in different clinical settings.