To evaluate risk factors for infection and the role of C-reactive protein (CRP) in the diagnosis of infection, a retrospective case control study was performed among Korean systemic lupus erythematosus patients. Of 120 proven infections, 31 episodes (25.8%) occurred in patients taking no corticosteroids (CS). The risk of infection was lower in patients taking low-dose CS (< 300 mg prednisolone/month) than no CS (odds ratio (OR) 0.36). In patients receiving high-dose CS (> 1000 mg prednisolone/month), however, the risk increased (OR 2.9). In patients taking no CS, disease activity manifested as increased SLEDAI, anemia and active urinary sediment, was associated with infection. The CRP was higher in the patients with infection than controls and the CRP levels over 50 mg/l were observed only in infection. These results suggest that CS have a bimodal influence on infection depending on dose. Disease activity is an important risk factor for infection in patients taking no CS. Finally, CRP levels greater than 50 mg/l suggest the presence of infection.