Background: The efficacy of azathioprine (AZA) and mycophenolate mofetil (MMF) for interstitial lung disease (ILD) has been described, but mainly in connective tissue disease-associated ILD. The objective of this study was to evaluate the effect of AZA and MMF on lung function and prednisone dose in myositis-related ILD (M-ILD). Methods: In this retrospective study, patients with M-ILD seen at Johns Hopkins and treated with AZA or MMF and no other steroid-sparing agents were included. Linear mixed-effects models adjusted for sex, age, antisynthetase antibody, and smoking status were used to compare the change in FVC % predicted, diffusing capacity of the lungs for carbon monoxide (DLCO) % predicted, and prednisone dose. Results: Sixty-six patients with M-ILD were treated with AZA and 44 with MMF. At treatment initiation, mean FVC % predicted and DLCO % predicted were significantly lower in the AZA group than in the MMF group. In both groups, FVC % predicted improved and the prednisone dose was reduced over 2 to 5 years; however, for DLCO % predicted, only the AZA group improved. The adjusted model showed no significant difference in posttreatment FVC % predicted or DLCO % predicted between groups (mean difference of 1.9 and –8.2, respectively), but a 6.6-mg lower dose of prednisone at 36 months in the AZA group. Adverse events were more frequent with AZA than MMF (33.3% vs 13.6%; P = .04). Conclusions: In M-ILD, AZA treatment was associated with improved FVC % predicted and DLCO % predicted, and lower prednisone dose. Patients treated with MMF had improved FVC % predicted and lower prednisone dose. After 36 months, patients treated with AZA received a lower prednisone dose than those treated with MMF.