A 31‐year‐old woman with systemic lupus erythematosus (SLE) was studied because she developed markedly reduced total hemolytic complement activity at a time when her disease was clinically inactive. Functional assays demonstrated reduced activities of C1, C4, and C2, but normal concentrations of C3 and the terminal (C5–9) components were present. Antigenic concentrations of C1, C4, and C2 were normal. Plasma or serum obtained from blood allowed to clot at 37°C had normal complement activity. Complement activity was depleted when the patient's serum was incubated in the cold. At reduced temperatures, the patient's serum (or purified IgG) depleted complement activity of normal human sera. A second patient with SLE was also demonstrated to have this same phenomenon. Cryoglobulins were not detectable in these patients. These data indicate that in vivo the patients' complement was normal and that the observed in vitro reduction was caused by cold dependent activation of the classical pathway.