The usefulness of serological parameters in assessing clinical exacerbations of SLE was examined. Patients with active renal disease tend to have lower levels of CH50 and C3 and highest levels of immune complexes detected by C1qBA than those patients with extrarenal manifestations only. Patients with a combination of both active extrarenal and renal disease are more likely to demonstrate the lowest levels of CH50, C4, and C3. However, immune complex levels are not higher than levels detected in patients with only active nephritis. A normal C3 level argues against active nephritis. Low complement levels without appreciably elevated levels of C1qBA suggest that significant renal disease is unlikely. The serial measurements that best reflect evolving clinical activity and which may serve as markers of impending exacerbation are, in decreasing order of usefulness: C4, CH50, C1qBA, C4, C3 and ADA. However, a combination of CH50, C4, C3 and C1qBA appeared to be the most useful. Given various serologic changes, guidelines for following patients are offered.