We measured the plasma levels of B-lymphocyte stimulator (BLyS) in 101 HIV-1-infected patients and 18 controls. BLyS levels were higher among HIV-positive patients [median 5.70 (3.90) versus 4.62 (1.04) ng/ml, P = 0.002], who had significantly higher BLyS and total serum globulin levels with decreasing CD4 cell counts. Moreover, BLyS levels increased exponentially below 100 CD4 cells/microl. BLyS and globulin levels increase as HIV dis ease progresses, suggesting a role for BLyS in the hypergammaglobulinemia of HIV infection.