The aim of this study was to determine whether the sensitivity and specificity of measuring CD8+ T lymphocyte percentages and numbers could be diagnostically useful in the assessment of polymyalgia rheumatica/giant cell arteritis (PMR/GCA). Double immunofluorescence and FACScan analysis were used to identify and quantify the CD3+CD8+ lymphocyte population. The absolute cell numbers were determined by a Coulter Counter. Random disease controls were used for comparison with 30 patients diagnosed as PMR/GCA. Both the percentage of CD3+CD8+ cells (17.4% PMR/GCA group vs 30.5% control) and the absolute numbers of CD8+ lymphocytes (310 cell/ml PMR/GCA vs 723 cells/ml control) were significantly reduced (p < 0.001) in the PMR group. The sensitivity of reduced CD8+ percentages for diagnosis was 73% and the specificity was 85%. Analysis of likelihood ratios showed that CD8+ lymphocyte percentages < 22.15% were five times more likely in patients with PMR/GCA than in controls. Data suggested that reduced CD8+ lymphocyte levels may be useful in the initial clinical assessment of patients with PMR/GCA. Only a larger study can establish whether these parameters should be used as diagnostic criteria.