We investigated the relationship between dermatomyositis/polymyositis (DM/PM) and the risk of subsequent osteoporosis development. A population-based retrospective cohort analysis was conducted using the National Health Insurance Research Database and the Catastrophic Illness Patients Database of Taiwan. We included 1179 patients and 4716 patients from 1999 to 2008 as the DM/PM cohort and the comparison cohort, respectively, and calculated the incidence rates of newly diagnosed osteoporosis. We used Cox proportional hazards models stratified on matched pair to assess the effect of DM/PM. The Kaplan-Meier method was applied to estimate the cumulative osteoporosis incidence curves. Patients with DM/PM were 2.99 times more likely to experience osteoporosis than those without DM/PM. The risk for osteoporosis in DM/PM patients was higher than comparisons in different propensity score quartiles. DM/PM cohort, no matter treated with or without corticosteroids and immunosuppressant, had a higher risk than the comparison cohort. The incidence of osteoporosis in Taiwan is associated with a priori DM/PM history. This risk was independent of the corticosteroids and immunosuppressant treatment.