Serial serum samples from 33 patients with systemic lupus erythematosus were tested for CH50, C3, C4, and circulating immune complexes. Circulating immune complexes were determined by 5 different assays. Disease activity was determined by a scoring system we devised. On an interpatient analysis, overall disease activity correlated significantly with levels of CH50, C3, and a positive result on C1q binding assay. However, with the exception of depressed C3 levels, the sensitivity, specificity, and predictive values of all the serologic tests were low. An intrapatient analysis revealed a patient-specific activity parameter in 11 of the 33 patients. We conclude that these serologic tests are not reliable in assessing disease activity in patients with systemic lupus erythematosus, although a small subgroup of patients will exhibit a patient-specific activity parameter.