Deposition of complement factors, immunoglobulins and infiltrating cells was evaluated by immunohistochemical staining in 30 temporal artery biopsy specimens from patients suffering from temporal arteritis and/or polymyalgia rheumatica and in controls. In the temporal arteritis group infiltrating cells, classic complement, alternative complement and lytic complex activation were detected. In specimens from patients suffering from only polymyalgia rheumatica there was unexpected evidence of classic complement and lytic complex activation. We conclude that immuno-histochemistry provides support for the concept of temporal arteritis and polymyalgia being based on the same pathological process.