Assay of anti-striated muscle antibodies (StrAb) is used as a diagnostic test for thymoma in individuals with myasthenia gravis. Unfortunately, false positive results are common, and their effect on test accuracy is magnified by the relatively low frequency of thymoma in myasthenics. We estimated the predictive values of positive and negative StrAb assays (PV+ and PV-) and gain in diagnostic certainty using values for sensitivity, specificity, and thymoma frequency previously reported in the literature. Sensitivity for indirect immunofluorescence assays ranged from 0.83 to 0.84 (pooled estimate = 0.84; n = 68). Specificity ranged from 0.73 to 0.88 (pooled estimate = 0.77; n = 898). Frequency of thymoma ranged from 0.07 to 0.17 (pooled estimate = 0.12; n = 4,515). The expected gain in diagnostic certainty from StrAb was minimal in those aged 0 to 20 and those aged 60+. In those aged 20 to 60, a negative test helped to exclude thymoma, and a positive test raised diagnostic suspicion but was not sufficient to establish a diagnosis of thymoma (PV+ = 0.46).