Peripheral blood lymphocyte subsets were determined by flow cytometry in 89 Chinese patients with polymyositis (PM) and dermatomyositis (DM). We aimed to investigate the clinical significance of peripheral blood lymphocyte subsets in PM/DM. Patients with active DM showed significant decreases in numbers of CD3(+) cells, CD3(+)CD4(+) cells, and CD3(+)CD8(+) cells, as compared to patients with inactive DM and healthy controls (P < 0.05). CD3(+) and CD3(+)CD4(+) cell counts were significantly lower in DM before treatment, compared with after treatment (t = -5.714 and -3.665, P < 0.05). Counts of CD3(+) cells, CD3(+)CD4(+) cells, CD3(+)CD8(+) cells, and CD19(+)CD5(-) cells were all correlated with the total disease activity score as determined by the Myositis Disease Activity Assessment Visual Analogue Scale (P < 0.05). The decreased number of CD3(+) cells and the decreased percentage of CD3(+)CD4(+) cells were additionally correlated with the presence of interstitial lung disease in PM/DM (P < 0.05). The presence of levels of CD3(+)CD8(+) cells was risk factor for death (b = -0.011, OR = 0.989, P < 0.05). The identification of peripheral blood T lymphocyte subsets in PM/DM appears to be useful as a reference marker in the evaluation of clinical disease activity, and be useful in the comprehensive assessment of clinical lung involvement. A decrease in CD8(+) T cells may predict a poor outcome in patients with PM/DM.