Type B1 thymoma is widely accepted as a tumor with a non-aggressive behavior even in advanced stage. Most of these tumors are classified as Masaoka stage I or II. They rarely relapse or metastasize and the surgical treatment is considered curative. We have investigated a case of thymoma type B1, which relapsed 13 months after the primary tumor was excised. The patient was diagnosed with a local tumor recurrence after investigations due to the worsening of clinical symptoms of myasthenia gravis (MG). The therapy management of such cases is debatable and protocols not yet approved. For this reason, we have analyzed different clinical, morphological and immunohistochemical characteristics that may be considered as prognostic factors for a more aggressive behavior of such tumors. We have identified some morphologic characteristics rarely seen in this type of thymoma but none considered of prognostic value. In addition, we investigated some possible immunohistochemical markers that are generally associated with a more aggressive clinical outcome in different malignant tumors and thymic epithelial tumors. Among these markers, only p53 was positive and may be useful to predict a more aggressive evolution. In summary, probably the more appropriate approach of the patient is the clinical follow-up together with treatment of the clinical symptoms of myasthenia gravis.