AbstractIn situ follicular B cell neoplasm, previously known as follicular lymphoma in situ, is a neoplastic proliferation of follicular lymphoma‐like B cells confined to the germinal centers. Herein, we report a case of a woman in her 70s who initially presented with several enlarged abdominal lymph nodes. Seven months later during follow‐up, a solitary pulmonary nodule was detected. As it was close to the hilum, lobectomy was performed. The intraoperative frozen section showed fibrosis and a collection of lymphocytes and macrophages. Therefore, the lymph nodes were sampled. Station 4 and 10 lymph nodes exhibited similar tumor cells and were immunohistochemically positive for CD10 and BCL2. Thus, the patient was diagnosed with in situ follicular neoplasm and is currently under observation. In situ follicular neoplasm is typically a slowly progressive neoplasm; however, it can present as a rapidly enlarging pulmonary nodule complicated by pulmonary aspergillosis.