This study aims to report the concomitant diseases observed and damage outcome in a cohort of patients with adult idiopathic inflammatory myositis (IIM) during long-term follow-up. All patients with IIM were identified from a single centre (follow-up between 1979 and 2006) and fulfilled at least three of the four Bohan and Peter criteria. Patients with inclusion body myositis, juvenile-onset myositis and overt overlap syndromes were excluded. Medical notes were retrospectively reviewed. Concomitant diseases identified were divided into 12 different organ systems (bone, cardiac, respiratory, gastrointestinal, renal, central nervous, malignancy, infection, endocrine, eyes, dermatological and haematological). Patient damage index was calculated using the Myositis Damage Index tool. Fifty-five patients (31 polymyositis, 24 dermatomyositis) were identified. The most prevalent organ system involved was lung with 40 events per 1,000 patient years follow-up. There was significant steroid-related complications with 17/18 patients with bone involvement having osteopenia/osteoporosis. Sjogren's syndrome (n = 3) was the most frequent concomitant auto-immune disease observed. Patients with a higher number of organ systems involved had a significantly higher damage index (r = 0.48, p = 0.001). White patients showed a significant trend to develop more than three other organ system involvement (p < 0.0001) and myositis-related lung disease (p < 0.0001) compared to other races. There is significant steroid-related morbidity in adult IIM patients under long-term follow-up. The prevalence of another concomitant auto-immune disease unlike patients with lupus or Sjogren's syndrome is low.