Lymphocytopenia has been reported in patients with connective tissue diseases, including dermatomyositis (DM). However, the risk of infectious complications and the changes of lymphocytic subsets during treatment have been poorly investigated in these patients. We investigated the alterations of peripheral blood lymphocyte counts in patients with DM. A retrospective analysis was conducted in patients with an ascertained diagnosis of DM admitted from 1994 to 2000 in both departments of Dermatology of the Saint-Louis Hospital in Paris. All patients had a peripheral blood absolute lymphocyte count available before therapy. From an initial set of 63 patients, 47 were included in the study. The median absolute lymphocyte count was 888/mm(3) (range, 400-4,070). Low peripheral blood CD4+ and CD8+ T-cell and B-cell counts were consistent findings (median CD4+: 382/mm(3); CD8+: 211/mm(3); CD19+: 122/mm(3)). There was a significant increase in lymphocyte count after 1 month (p < 0.0001), 3-6 months (p = 0.001), and 6-12 months (p = 0.0005) of corticosteroid treatment. Infectious events, mainly pneumonia (PCP), occurred in 12 patients. Their initial lymphocyte count was lower than that of patients who did not develop infections (p = 0.0001). These results support the high prevalence of lymphocytopenia in patients with DM and emphasize the risk for opportunistic infections, mainly PCP, in these patients. Further studies are warranted to evaluate the risk/benefit balance of PCP prophylaxis in patients with DM and severe lymphocytopenia.