CRP levels in 194 serum samples from 43 SLE patients were measured. Patients with inactive disease have levels below 10 micrograms/ml; patients with active SLE have higher levels, but never over 50 micrograms/ml. In the presence of infection or inflammatory processes, regardless of the activity of SLE, the levels are significantly higher (p less than 0.05), and well over 50 micrograms/ml. Both active SLE patients and inactive SLE patients with local infections have levels between 10 micrograms/ml and 50 micrograms/ml. In this situation, the presence of anti-DNA antibodies strongly suggests disease activity (82% versus 9%, p less than 0.05). The clinical and physiopathological meaning of these findings is discussed.