Myasthenia gravis (MG) is caused by autoantibodies to the acetylcholine receptor (AChR), but several other muscle autoantibodies have also been identified in patient sera. We studied muscle autoantibodies against AChR, striated muscle tissue sections (SH), titin, citric acid antigen (CA), and ryanodine receptor (RyR) in sera from 146 consecutive MG patients to evaluate whether a single test or several tests together can predict a thymoma. The MG patients were divided into five subgroups; ocular MG, early-onset MG (< 50 years), late-onset MG (> 50 years), MG with thymoma, and AChR antibody negative MG. AChR, SH, titin, CA, and RyR antibodies were detected in 85%, 34%, 34%, 25%, and 14% of the MG patients, respectively. For thymoma MG, AChR, SH, titin, CA, and RyR antibodies were detected in 100%, 75%, 95%, 70%, and 70% respectively. SH, titin, CA, RyR antibodies, and computed tomography of the anterior mediastinum have similar sensitivity for thymoma MG. The specificity of RyR, titin, CA, and SH antibodies for thymoma was 70%, 39%, 38%, and 31%, respectively, which is significantly higher for RyR antibodies than for the others. No single muscle antibody assay can predict a thymoma, and a combination of several antibody assays is preferred, although RyR antibody testing alone showed 70% sensitivity and specificity for thymoma MG. SH and CA antibodies provided only little additional information.