Summary. We analysed the clinical features, course and response to immunosuppressive therapy in 30 patients with autoimmune disorders associated with myelodysplastic syndromes (MDS). 18 patients with MDS developed acute systemic autoimmune disorders. Common manifestations were skin vasculitis (n=15) and arthritis (n=11). Seven patients had an acute clinical syndrome of vasculitic skin rash, fever and arthritis with peripheral oedema in three and pulmonary infiltrates in five of these seven patients. Other acute manifestations included pericarditis, pleural effusions, skin ulceration, seizures, myositis and peripheral neuropathy. Chronic or isolated autoimmune manifestations (n=11) included glomerulonephritis, polyneuropathy, pyoderma gangrenosum, ulcerative colitis and polyarthritis. Classic connective tissue disorders recognized included relapsing polychondritis, polymyalgia rheumatica, Raynaud's syndrome and Sjogren's syndrome. Autoimmune manifestations responded to immunosuppressive therapy (primarily prednisone) in 26/27 patients treated. Furthermore, cytopenias improved substantially in six patients, including complete normalization of peripheral blood counts in two patients with cytogenetic remission in one. Patients with a haematological response to immunosuppressive therapy had improved survival compared with non-responding patients. The autoimmune syndrome was implicated as a primary cause of death in 8/17 patients who died. Autoimmune manifestations may be more common than previously recognized in patients with MDS. Aggressive therapy with immunosuppressive agents in selected patients often controls autoimmune phenomena associated with MDS and may lead to haematological responses in some patients.