Objective. To investigate the associations between anti-nuclear matrix protein 2 (anti-NXP-2) autoantibody levels and disease activity as well as calcinosis severity in patients with idiopathic inflammatory myopathies (IIMs). Methods. Serum levels of anti-NXP-2 autoantibodies were determined in 709 patients with IIMs and also serially measured in the patients' sera with an in-house enzyme-linked immunosorbent assay using MORC3 recombinant protein. Patients with anti-NXP-2 autoantibodies were divided into 2 subgroups: those with and those without calcinosis. Associations of anti-NXP-2 autoantibody levels with organ-specific disease activity (using 10-cm visual analog scale [VAS] scores), serum creatine kinase (CK) levels, and calcinosis severity were investigated in cross-sectional and longitudinal analyses. Results. A cross-sectional analysis of 56 IIM patients with anti-NXP-2 autoantibodies (38 without calcinosis and 18 with calcinosis) showed that in patients without calcinosis, the levels of anti-NXP-2 autoantibodies were positively correlated with the physician's global assessment of disease activity and muscle VAS scores and serum CK levels, whereas no such association was found in patients with calcinosis. Results of the longitudinal study revealed strong correlations of anti-NXP-2 antibody levels with the physician's global assessment and constitutional, cutaneous, gastrointestinal, and muscle VAS scores and serum CK levels in patients without calcinosis, but in patients with calcinosis, only a moderate correlation was observed between anti-NXP-2 antibody levels and the physician's global VAS and constitutional VAS scores. Of note, in patients without calcinosis, anti-NXP-2 autoantibodies were found to disappear during periods of clinical remission, but reappeared with disease relapse. No association between anti-NXP-2 antibody levels and the severity of calcinosis was observed. Conclusion. These findings indicate that anti-NXP-2 autoantibodies serve as a useful marker for disease activity in patients with IIMs, especially in the absence of calcinosis. The differential associations observed between anti-NXP-2 autoantibody levels and disease activity suggest that there may be a phenotypic difference between patients with and those without calcinosis.