The numbers of total lymphocyte and lymphocyte subsets in peripheral blood of patients with silicosis and their serum immunoglobulin (Ig) levels were studied to evaluate their immune status. The numbers of total lymphocytes and OKT4+ and OKT8+ cells tended to be decreased and the numbers of OKT3+ cells was significantly decreased (p less than 0.001), but that of OKIa-1+ cells was increased (p less than 0.001) in the patients. In the lymph nodes of the lung hilus, the ratio of OKT4+ cells to total lymphocytes was increased but that of OKT8+ cells to total lymphocytes was decreased. The serum levels of IgG and IgA were elevated in the patients with reduction in the numbers of total lymphocytes and OKT4+ cells in peripheral blood. The serum IgE levels in 17 of 82 patients were above 400 IU/ml. In 8 patients with serum IgE 82 patients were above 400 IU/ml. In 8 patients with serum IgE levels of more than 1,000 IU/ml, the numbers of total lymphocytes and OKT3+ and OKT4+ cells were decreased compared with those in controls. The following suggestions are proposed from the results: Increase and activation of helper T cells may be caused by the presentation of silica dust to macrophages in the lymph nodes, and these cells may then stimulate B cell proliferation and Ig production by B cells, resulting in increase in the serum Ig (IgG, IgA and IgE) level. Reduction in the number of OKT4+ cells helper/inducer T cells) in peripheral blood may be due to migration of helper T cells into the lymph nodes. These immunological events in patients with silicosis are probably due to the adjuvant effect of silica dust.