OBJECTIVES--Some reports have described a decreased percentage of circulating CD8+ cells in patients with polymyalgia rheumatica and giant cell arteritis (PMR/GCA) before treatment and persisting for some months during treatment with corticosteroids. Other studies have found no such changes. There are overt methodological variations between these studies and there may also hidden differences, such as the timing of blood samples. The purpose of this study was to investigate T cell subtypes in patients with PMR/GCA while controlling for variables known to affect T cells. METHODS--Circulating T cell subsets were measured in 36 patients with PMR/GCA before and during treatment with prednisolone. Blood samples during treatment were taken before the daily dose of prednisolone. The whole blood lysis method was used followed by flow cytometry. RESULTS--Compared with controls, CD8+ cells were not reduced before treatment in patients with PMR/GCA (0.44 x 10(9)/l; 28% of lymphocytes). CD4+ cells were also normal (0.78 x 10(9)/l; 48% of lymphocytes). During treatment with prednisolone total T cells increased from 1.18 to 1.59 x 10(9)/l and CD4+ cells increased from 0.78 to 1.05 x 10(9)/l. The percentage of CD8+ cells decreased on treatment from 28 to 25%. CONCLUSIONS--This study does not confirm the finding of some groups that the percentage of circulating CD8+ cells is reduced in patients with PMR/GCA before treatment. It does show that the percentage of CD8+ cells decreases during treatment with corticosteroids. This needs to be considered when designing studies of lymphocyte subsets in diseases treated with corticosteroids.