The aim of this work is to evaluate disease-related mortality and the course of the disease including functional outcome and quality of life. We did a follow-up study on a large prospective cohort of 62 patients with subacute-onset idiopathic inflammatory myopathy (IIM) (dermatomyositis (n = 24), nonspecific myositis (n = 34), necrotizing autoimmune myopathy (n = 4)) after treatment with corticosteroids only (randomized controlled trial comparing daily high-dosage prednisone with pulse therapy of dexamethasone). Development of connective tissue disease (CTD) or malignancy, disease course and mortality, functional outcome and quality of life were evaluated. After a mean follow-up of 3 years (SD 1.5), 22 % had developed a CTD and 17 % a malignancy. Disease-related mortality was 15 %. A monophasic disease course was found in 27 %. Most patients had a chronic (35 %) or polyphasic disease (35 %) course and experienced single or multiple relapses. Sixteen patients (33 %) were off medication after a mean of 1 year of treatment. Disability scores improved particularly in the first 18 months. At follow-up, 68 % still perceived disabilities. Quality of life scores as measured by the short-form (SF)-36 improved in the first 18 months. After 18 months, scores remained stable during the next years of follow-up and remained low compared to a normal population. (1) Two-thirds of the patients with an IIM have a polyphasic or chronic disease course and need maintenance treatment. (2) The impact on functional outcome and quality of life is considerable and does not improve further after 18 months.