Background: There is a paucity of literature on the patterns and predictors of mortality in idiopathic inflammatory myopathies (IIM). Aims: To determine the patterns and predictors of mortality in a South Australian cohort of patients with biopsy-proven IIM. Methods: The living/deceased status (and for deceased patients the causes of death) of patients with histologically determined IIM was determined from the Births, Deaths and Marriages Registry. Standardised mortality ratios (SMR) were generated compared with the age/gender matched South Australian population. The effect of presence/absence of the components of the Bohan and Peter criteria on risk ratios (RR) for mortality was determined. The effect of comorbidities and autoantibodies on mortality was investigated. Results: The SMR for mortality in IIM was 1.75 and was significantly increased in all disease subgroups, being highest in patients with dermatomyositis (2.40). Dominant causes of death were cardiovascular disease (31%), infections (22%) and malignancy (11%). Risk factors for death were age at time of biopsy (hazard ratio 1.05), ischaemic heart disease (RR 2.97, P < 0.0001), proximal weakness at diagnosis (RR 1.8, P= 0.03), definite diagnosis of IIM per the Bohan and Peter criteria (RR 2.14, P < 0.0001), and the absence of autoantibodies (RR 1.9, P < 0.001). Conclusions: Patients with IIM are at 75% increased risk for mortality, and cardiovascular diseases account for the commonest causes of death. This study suggests a thorough cardiovascular evaluation of these patients is indicated, and raises the possibility that targeted interventions such as the use of aspirin or statins may improve outcomes in IIM.