Serum immune complexes were measured in 22 patients with eosinophilic fasciitis, 8 of whom had serial determinations. Elevated levels were found by Raji cell radioimmunoassay in 14 (64%) patients, by agarose gel electrophoresis in 13 (59%), and by C1q agglutination-inhibition in 9 (41%). Elevated levels by Raji cell assay were present more frequently at times of active disease [17 of 22 sera (77%)] than at times of inactive disease [2 of 24 (8%) (P less than 0.0005)] and were more closely correlated with disease activity than were eosinophilia, hypergammaglobulinemia, or increased erythrocyte sedimentation rate.