Idiopathic inflammatory myopathies/IIM are associated with muscle atrophy and metabolic dysfunction. Regular exercise improves muscle function and patients’ clinical state. Limited evidence points at low circulating vitamin D/vitD levels in IIM patients, which could contribute to metabolic dysfunction. Previously, we showed that altered lipid metabolism in muscle cells from IIM patients was normalized by 6-month training. Our aim was to assess (i) total & active/A (1,25-diOH) vitD in serum of patients with IIM before & after training intervention and of healthy controls, and (ii) associations between circulating vitD and lipid metabolism in patients’ cultured muscle cells. IIM patients underwent 6-month training (supervised training of activities of daily living 2x/week + individual home-based exercise). Serum total & A-vitD were assessed. Muscle Bergstrom biopsy (vastus lateralis) was performed in IIM patients before & after training (5F/2M, age 52.7±9.6 yrs, BMI 27.5±10.6 kg/m2) and in age/gender/BMI-matched sedentary healthy controls. Muscle cell cultures were established and exposed to 100µM palmitate (3 last days of differentiation). After 5 days of differentiation, vitD receptor mRNA (VDR, q-PCR), production of CO2 & β-oxidation intermediates (fat oxidation/FOx assay with [1-14C]-palmitate), and incorporation of labeled palmitate into lipids (thin layer chromatography) were measured in myotubes in vitro. Compared to controls, IIM patients had higher total and lower A-vitD levels before (p=0.055 & 0.009) and after (p=0.044 & 0.006) training. Training did not change serum vitD in IIM patients. VDR mRNA in myotubes was not modulated by IIM or training. However, 3-day palmitate tended to reduce VDR mRNA in myotubes in vitro (IIM before training & controls: p=0.06). Serum A-vitD was negatively associated with palmitate-induced changes in VDR mRNA (R=-0.598, p=0.011) and positively with incomplete FOx (R=0.587, p=0.005), total fatty acid/FA disposal (R=0.656, p=0.001), and % oxidative disposal from total FA disposal (R=0.573, p=0.007) in myotubes. Total vitD serum levels correlated negatively with incorporation of FA into diacylglycerols (R=-0.535, p=0.012) in myotubes. Patients with IIM displayed lower active and higher total vitamin D serum levels. Six-month training did not modulate vitamin D in serum and expression of its receptor in myotubes. Relationships of serum vitamin D levels with the expression of its receptor and parameters of lipid metabolism in muscle cells in vitro suggest the importance of vitamin D for muscle metabolic regulation in patients with IIM.