To evaluate a flare of systemic lupus erythematosus (SLE) during pregnancy and to differentiate it from diseases of pregnancy, serological parameters are often utilized. However, there are conflicting reports regarding the merit of conventional measurements of complement and activation products. While in normal pregnancy the levels of serum C3, C4, and CH50 gradually rise, a decline in these levels occurs during the course of pregnancy in selected SLE patients. There is controversy regarding whether such falls represent decreases in the overall synthesis of complement or activation, the former theory being supported by a report of normal levels of the C1s-C1 inhibitor complex. During normal pregnancies, increases of complement split products, such as plasma C3a, may occur, and these correlate positively with elevations of C3. In pregnancies complicated by lupus, increases of C3a are often accompanied by a decline in total C3 and CH50. In a minority of non-SLE patients, preeclampsia has been associated with elevations of a variety of complement split products. Ba, C3a, C4d, SC5b-9, indicating activation of both the classical and alternative pathways. The CH50 levels tend to remain normal in these patients. In contrast, elevations of complement split products frequently accompany disease flares in patients with SLE. A high ratio of CH50/Ba may differentiate patients with preeclampsia from those with active SLE. A decline in conventional measures of C3, C4, or CH50 which is accompanied by elevations of complement split products appears to differentiate a lupus flare from non-SLE diseases of pregnancy.