Interleukin-21 (IL-21) is the most recent member of the common gamma-chain-dependent cytokine family. We studied the expression of the IL-21 receptor (IL-21R) on peripheral B lymphocytes in patients with systemic lupus erythematosus (SLE) or primary Sjögren's syndrome (pSjS), and healthy controls. Naive B lymphocytes expressed higher levels of IL-21R than memory B lymphocytes and plasmablasts, both in SLE patients and healthy controls. The proportion of IL-21R+ cells in the total peripheral B lymphocytes, as well as those in the respective B lymphocyte subsets, was significantly lower in SLE compared with those of pSjS or healthy controls (p=0.0002 and p<0.0001, respectively, in total B lymphocytes). The decreased expression of IL-21R in SLE was significantly associated with nephritis and high titer anti-double-strand DNA antibody. In some SLE patients, IL-21-induced proliferation of CD19+ B lymphocytes, in the presence of CD40 stimulation, was impaired. The abnormalities of IL-21R signaling might contribute to the pathological features of SLE, such as B lymphocytopenia.