Background Imaging findings of both anterior cruciate ligament (ACL) sprain and mucoid degeneration overlap in some cases, which may cause errors in magnetic resonance imaging (MRI) evaluation. Purpose To determine the ancillary findings on MRI in distinguishing between ACL sprain and mucoid degeneration, and also to obtain a diagnostic scheme. Material and Methods MRI scans of 77 patients with ACL mucoid degeneration and 77 cases with ACL sprain were retrospectively evaluated to compare with regard to parameters of age, sex, side, the status of posterior cruciate ligament – medial collateral ligament – lateral collateral ligament, bone marrow edema, intraosseous cyst, subchondral sclerosis, chondromalacia, meniscus tear, effusion, and osteochondral body. A decision tree algorithm was created to predict pathology in ACL, whether it was a sprain or mucoid degeneration. Results The prevalence of female sex, femoral intraosseous cyst, tibial intraosseous cyst, subchondral sclerosis, femoral chondromalacia, tibial chondromalacia, medial meniscus tear, and lateral meniscus tear were significantly higher in the ACL mucoid degeneration group ( P < 0.001, P = 0.016, P < 0.001, P = 0.003, P < 0.001, P < 0.001, P < 0.001, and P < 0.001, respectively). The probability of being mucoid degeneration increased 41.2 times (95% confidence interval [CI] = 5.296–321.132) in cases with tibial intraosseous cyst and increased 1.05 times (95% CI = 1.010–1.080) with each one-year increase in age ( P < 0.001 and P = 0.011, respectively). The decision tree algorithm had an overall accuracy of 79.2%. Conclusion Ancillary findings are helpful in the diagnosis of suspicious cases for ACL mucoid degeneration and ACL sprain. The decision tree algorithm offers a practical and successful approach to this issue.