Aim: To review the epidemiology of dermatomyositis (DM) in South Australia (SA) and to compare it with other Australian states and New Zealand (NZ). Methods: Muscle biopsy and hospital separation data for DM in SA, other Australian states, and NZ were determined. The role of environmental factors was investigated. Results: From 1990 to 2005, there were 21 cases of biopsy-proven DM in SA (62% F, mean age 49.7 ± 18.4) and 99 cases of polymyositis (PM). Based on biopsy-proven figures, the average incidence of DM per year in SA was 1.4 ± 1.2, and 6.6 ± 2.6 for PM. Since 1991, there were 221 and 441 total separations from SA hospitals with principal diagnoses of DM and PM, respectively. The ratio DM/DM + PM is thought to correlate with solar irradiance, and within Australia, SA had the lowest ratio (0.39, 95% CI 0.22-0.56), with the highest ratio seen in WA (0.67, 95% CI 0.53-0.81). This ratio did not correlate with latitude, duration of sunshine, cloud cover, relative humidity or total rainfall. Within SA, no correlation with socioeconomic status was seen. Australian data were similar to NZ, where the ratios were 0.34 and 0.3 for North and South Islands, respectively. As separation data reflect total hospital visits, we also ascertained individual patient separations from SA hospitals (1997 to July 2005) and found a similar ratio of DM/DM + PM (0.38 ± 0.08). Conclusions: The proportion of inflammatory myositis which is DM varies nationwide, with a consistent ratio seen in SA (33-38%). Geoclimatic variables do not appear to influence DM/PM disease expression in Australia.