Polymyalgia rheumatica and giant cell arteritis are diseases which are characterized by a brisk acute phase response. Cytokines, principally interleukin-1 and interleukin-6, are thought to mediate the release of acute phase proteins from hepatocytes. Employing a sensitive bioassay based on the hybridoma growth factor properties of interleukin-6, we investigated its levels in sequential sera obtained from 15 initially untreated patients with PMR/GCA. We found that interleukin-6 activity was significantly elevated in all untreated patients. Although there was a significant decline in its levels after successful steroid therapy, seven patients continued to have raised serum levels of interleukin-6 for up to 6 months after initiation of treatment. Activated endothelial cells are a potent source of interleukin-6 and its persistence in the circulation may be taken as evidence of continuing disease activity. What is not explicable is the normalization of the acute phase response soon after the commencement of steroid therapy when circulating levels of interleukin-6 are still high. This suggests that steroids may be having additional effects on hepatocyte function.