Objective The long-term outcomes of juvenile dermatomyositis (JDM) are more favorable in recent years. However, calcinosis is still among the complications that can cause serious functional impairment. Little is known about the pathogenesis and risk factors of calcinosis. The aim of this study is to determine risk factors for the development of calcinosis in JDM. Methods This was a single-center, retrospective cohort study. All patients were diagnosed and followed at the multidisciplinary JDM clinic of The Hospital for Sick Children, from January 1, 1989, until May 31, 2018. To investigate predictors of incident calcinosis, Cox regression analysis was performed. Results A total of 172 patients met inclusion criteria, with a median age at diagnosis of 7.7 years (IQR 4.9-12.1), and a median follow-up of 8.5 years (IQR 3.4-12.6, range 0.1-28.3). The only risk factor significantly associated with the development of calcinosis in the univariate analysis was nailfold abnormality at baseline (hazard ratio [HR] 4.86, P = 0.03). In multivariable analysis, including nailfold abnormality, age of diagnosis, sex, and duration from onset to diagnosis, the only statistically significant risk factor for calcinosis was the presence of nailfold abnormalities (HR 4.98, P = 0.03). Further, calcinosis was significantly increased in patients with a chronic course (chi-square 25.8, P < 0.001). Conclusion The presence of abnormal nailfold capillary changes at baseline is predictive for the development of calcinosis in children with idiopathic inflammatory myopathies.