To estimate the incidence rate and prevalence of idiopathic inflammatory myopathies (IIMs) in Sweden across clinical subgroups, age, sex, educational level and place of residence and to assess the robustness of register-based case definitions. IIM was identified from the Swedish National Patient Register and the Swedish Rheumatology Quality Register. The base case definition required ⩾2 visits indicating IIM (first ever with a consecutive visit within 1-12 months for incident cases) and the robustness was tested by applying a more liberal and a stricter definition. Using the base case definition, 558 incident IIM patients were identified between 2007 and 2011. The incidence was estimated to 11 (13 for women and 9.7 for men) per 1 000 000 person years and was stable across case definitions. Incidence increased with age and peaked at the 50-79 years age groups. No differences were observed between different levels of education and place of residence. We identified 1267 IIM patients on 1 January 2012 corresponding to a prevalence of 14 per 100 000. We present nationwide register-based incidence and prevalence estimates for IIM, robust across three different case definitions. In contrast to many other reports, we did not find incidence by age to be bimodal and we found no explanation of incidence variation across education and residency. These register-based case definitions can be included in future population-based studies to better understand disease aetiology, risk factors and comorbidities.