Background  Nationwide data on the epidemiology of dermatomyositis (DM) and polymyositis (PM) were limited.

Objectives  This study was to estimate the incidence, occurrence of cancer and mortality of DM and PM in Taiwan.

Methods  Both the register of critical illness of the Taiwan National Health Insurance Research Dataset and the National Death Registry of Taiwan were used to calculate estimates of the incidence, cancer association, and mortality of DM and PM between 2003 and 2007.

Results  A total of 803 DM and 500 PM cases were identified between 2003 and 2007. Mean age at diagnosis was 44·0 ± 18·3 years for DM and 49·2 ± 15·9 years for PM. The overall annual incidences of DM and PM were 7·1 (95% CI 6·6–7·6) and 4·4 (95% CI 4·0–4·8) cases per million population. The incidence of both DM and PM increased with age and reached a peak at age 50–59 years. One hundred and eleven (13·8%) patients with DM and 31 (6·2%) patients with PM had cancers. The diagnosis of most cancers was made after the diagnoses of DM (n = 71; 64·0%) and PM (n = 21; 67·7%). Overall, the standardized incidence ratios (SIR) for cancer were 5·36 (4·12–6·87) and 1·80 (1·10–2·79) among patients with DM and PM; however, during the first year, SIRs for cancer were 24·55 (95% CI 18·62–31·79) and 9·17 (95% CI 14·82–15·93) in patients with DM and PM, respectively. The most common types of cancer were nasopharyngeal cancer for men and breast cancer for women. Patients with DM and PM had standardized mortality ratios of 7·68 (6·41–9·01) and 5·29 (4·28–6·48).

Conclusion  This study reports robust estimates of important aspects of the epidemiology of both DM and PM in Taiwan. This highlights the rarity of these diseases, and their associated cancer risks and increased mortality.