Objective: To study the relationship between different disease-related variables and bone mineral density (BMD) in patients with idiopathic inflammatory myopathies (IIMs).Method: Demographic and clinical data were retrospectively collected from the medical records of all patients diagnosed with IIMs during 2003-2018 in the Rheumatology Department, Sahlgrenska University Hospital, Gothenburg, Sweden. BMD measurements by dual-energy X-ray absorptiometry (DXA) were compared among three patient groups categorized according to the time when DXA was performed in relation to the diagnosis: during the first month, 2-6 months, and 7-24 months after diagnosis.Results: In total, 48 patients were included in the study. BMD correlated positively with body mass index and the presence of myositis-specific autoantibodies. As expected, age and diseases duration had negative associations with BMD. Importantly, osteopenia and osteoporosis were significantly more common in patients who underwent DXA at later time-points of the disease than in those who underwent DXA during the first month after diagnosis.Conclusions: Reduced BMD is common in patients with IIMs. The development of osteopenia/osteoporosis starts in the early phase of myositis (within 6 months), and immediate osteoporosis prophylaxis at diagnosis is necessary.