Median percentage and number of circulating CD8+ T lymphocytes (suppressor/cytotoxic T lymphocytes) are markedly decreased in patients with giant cell arteritis (GCA). We assessed the diagnostic usefulness of CD8+ T cell values, in 108 patients who had a temporal artery biopsy during a 3 year period. Histologic evidence of GCA was found in 26 patients. Negative biopsy patients were divided into 2 groups, one with 49 patients and a probable GCA syndrome (polymyalgia rheumatica with a negative biopsy) and a group with non-GCA (33 patients). Median percentage and number of CD8+ T cells were significantly decreased with definite GCA (CD8+% 10.0, CD8+ cells 0.221 x 10(9)/l), and probable GCA syndrome (CD8+% 10.0, CD8+ cells 0.197 x 10(9)/l) compared with non-GCA (CD8+% 22.0, CD8+ cells 0.527 x 10(9)/l) and controls (CD8% 21, CD8+ cells 0.518 x 10(9)/l). Significantly low CD8+ T cell values were found in nearly 80% of patients with active untreated GCA or GCA syndrome and in 15% of patients with non-GCA. Positive predictive value of a significant low CD8+ T cell value is 85% while the likelihood of no GCA or GCA syndrome with normal CD8+ T cell values is 60%.