Many immunological abnormalities, including circulating immune complexes (CICs), have been described in patients with sarcoidosis. The relationship of these abnormalities to clinical states is uncertain. In 31 proven cases of sarcoidosis we used the Raji assay in the investigation of the relationship between disease activity and the presence of CICs. Circulating immune complexes (greater than 49 micrograms.ml-1) were found in 11 of 14 patients with evidence of recent significant disease deterioration, whereas only one of 17 patients with chronic stable sarcoidosis demonstrated CICs (p less than 0.001). At follow-up, it was found that eight of the 11 cases with active disease and detectable CICs had resolved clinically. Resolution was accompanied by normalization of CICs in seven and persistence in one. The remaining three patients did not improve clinically and continued to demonstrate CICs. Measurements of complement levels did not demonstrate any significant difference between the two groups of patients. It is concluded that elevated levels of CICs, as detected by the Raji assay, are commonly found in association with significantly deteriorating sarcoidosis, often normalize following disease improvement and, consequently, may have a useful clinical role as a marker of activity.