Adenosine deaminase (ADA) and its isozyme activities in serum were measured together with peripheral lymphocyte subsets in 42 patients with idiopathic Parkinson's disease. The total and ADA 2 activities were significantly higher than normal controls (p < 0.01). As regards the peripheral lymphocyte subsets, the proportion of OKT 10+ cells (activated T lymphocytes) and the proportions of interleukin-2 receptor+ and HLA-DR+ cells (mainly activated T lymphocytes) were significantly higher than normal controls (p < 0.05, 0.01, 0.01, respectively). On the other hand, OKT 10+ cells demonstrated a significant correlation not only with total ADA but also with ADA 2 activity. These results suggest that high serum ADA activity may be involved in the pathogenesis of Parkinson's disease through peripheral T lymphocyte activation.