Aberrations of cellular immune functions in pyoderma gangrenosum (PG) may lead to nonspecific activation of inflammatory cells or to an imbalance of suppression leading to autoaggression (chronic ulceration). A patient with severe unremitting PG had anergy to a battery of seven skin test antigens. Mixed lymphocyte reactions, autologous mixed lymphocyte reactions, lymphocyte proliferative responses to antigens, and the production of leukocyte inhibitory factor were substantially suppressed, while the lymphocyte responses to mitogens were unaffected. Quantitative immunoglobulin and complement levels were normal. The inhibition of cellular immune functions was mediated by a factor in the patient's serum. This factor also inhibited lymphocyte functions of normal unrelated control subjects. Preliminary studies demonstrated that the factor is nondialyzable, heat stable, and not adsorbed by Staphylococcus A protein. Pulse therapy with large doses of corticosteroids resulted in dramatic clinical improvement.