Spindle cell thymomas with prominent amount of lymphocytes are classified as WHO type AB tumors. However, there are architectural pattern differences in these tumors. We investigated the importance of architectural pattern in type AB thymomas in relation to prognostic value. Archival hematoxylin-eosin stained slides of 37 AB type thymomas were reviewed for the presence (type 1) or absence (type 2) of reticular growth pattern. Reticular growth pattern is defined as the presence of a network of elongated bland spindle cells separating nests of tumor cells admixed with lymphoid cells. The architectural patterns were correlated with tumor stage at diagnosis and presence or absence of recurrent disease. The analysis identified 18 cases of type 1 AB thymoma and 19 cases of type 2. Type 2 cases also had greater cytologic atypia within the spindle cells. Patients with type 1 tumors were more likely to have early stage disease. In contrast, type 2 pattern was associated with higher stage at diagnosis (P<0.001) and greater likelihood for recurrence (P<0.05) and metastases. Architectural features are prognostically relevant in classification of WHO AB type thymomas and may constitute a form of personalized medicine. Independent confirmation of the findings is necessary to confirm the association of architectural pattern with outcomes.