The influence of adult thymectomy on several parameters of immunocompetence in patients with myasthenia gravis (MG) was investigated. Since incomplete thymectomy may lead to the presence of thymic remnants, we determined the activity of a thymus-dependent factor in the sera of the MG patients. As measured by several parameters, MG patients showed a normal immunocompetence compared with healthy controls, except in the response to DNCB sensitization in vivo. When tested at least 5 years after thymectomy, MG patients were found to have decreased response to mitogens, and a decreased cytotoxic T cell response in cell-mediated lympholysis. The response to challenge with DNCB in vivo was decreased both in thymectomized and nonthymectomized MG patients. No difference was found in a) the percentage of circulating T, B, non-B/non-T cells; b) the response to allogeneic cells (MLR); c) the antibody-dependent lymphocytotoxicity; d) the production of immunoglobulins in vitro by pokeweed mitogen-stimulated cells; and e) the anamnestic response to antigens in vitro. We conclude that adult thymectomy results in a decrease in the function of some subpopulations of lymphocytes.