Twenty-seven patients with SLE of juvenile onset were studies for 2 years. Episodes of active disease and quiescence were defined and were related to levels of anti-dsDNA and C3. Two methods for the detection of anti-dsDNA--the Farr assay and Cr immunofluorescence--were compared. The latter method was also used to differentiate anti-dsDNA according to its Ig class and its CF property. Positive tests for anti-dsDNA and low C3 levels were correlated with activity of the disease. Results with the Farr assay and Cr Ig test were comparable, but both low C3 and Cr CF anti-dsDNA showed a significantly stronger association (p less than 0.001) with active SLE than did DNA binding by the Farr method or Cr Ig anti-dsDNA. Furthermore, in six patients followed during active disease and remission, a negative Cr CF test was the earliest sign of ensuing clinical remission; DNA binding and C3 levels took weeks to months longer to normalize. This predictive value of CF anti-dsDNA may be useful in monitoring therapy for SLE, especially if symptoms due to prior renal damage may be confused with active disease as in lupus nephritis.