Subscapularis (SSC) lesions are often underdiagnosed in the clinical routine. This study establishes and compares the diagnostic values of various clinical signs and diagnostic tests for lesions of the SSC tendon. Fifty consecutive patients who were scheduled for an arthroscopic subacromial or rotator cuff procedure were clinically evaluated using the lift-off test (LOT), the internal rotation lag sign (IRLS), the modified belly-press test (BPT) and the belly-off sign (BOS) preoperatively. A modified classification system according to Fox et al. (Type I-IV) was used to classify the SSC lesion during diagnostic arthroscopy. SSC tendon tears occurred with a prevalence of 30% (15 of 50). Five type I, six type II, three type IIIa and one type IIIb tears according to the modified classification system were found. Fifteen percent of the SSC tears were not predicted preoperatively by using all of the tests. In six cases (12%), the LOT and the IRLS could not be performed due to a painful restricted range of motion. The modified BPT and the BOS showed the greatest sensitivity (88 and 87%) followed by the IRLS (71%) and the LOT (40%). The BOS had the greatest specificity (91%) followed by the LOT (79%), mod. BPT (68%) and IRLS (45%). The BOS had the highest overall accuracy (90%). With the BOS and the modified BPT in particular, upper SSC lesions (type I and II) could be diagnosed preoperatively. A detailed physical exam using the currently available SSC tests allows diagnosing SSC lesions in the majority of cases preoperatively. However, some tears could not be predicted by preoperative assessment using all the tests.