Multilocular thymic cysts (MTCs) are considered to be acquired lesions associated with various inflammatory conditions and/or malignant tumors. MTCs associated with thymomas are rare, with only 11 cases having been reported. On reviewing 110 consecutive patients with thymomas, we found 20 cases of MTCs. The patients included 18 men and 2 women aged 32 to 65 years (median 52 y). Eleven of the patients were symptomatic, and 6 presented with symptoms associated with inflammation. Computed tomography images were available for 11 patients, and cystic lesions were identified in 4 patients. The histologic subtypes of thymoma observed were: 3 tumors of type AB, 4 tumors of type B1, 9 tumors of type B2, and 4 tumors of type B3. In addition, 2 tumors were in advanced stages. Multilocular cystic structures accompanied by acute and chronic inflammation were observed in the remnant thymic tissues. Immunohistochemically, CK13 was diffusely expressed in the inner surface cells lining the cysts, whereas CK5/6 and p63 were primarily expressed in the basal cells of the cysts. D2-40 was weakly expressed in a small number of basal epithelial cells. The immunohistochemical profiles of the cysts were similar to those of Hassall corpuscles of normal thymi. A clinical follow-up showed that 15 patients continued to be alive without any evidence of disease, 1 patient with tumor recurrence continued to be alive, and 3 patients had died of other diseases. Our results suggest that MTCs associated with thymomas are not as uncommon as thought and may develop from the promotion of differentiation of increased numbers of epithelial cells into Hassall corpuscles by inflammatory processes. Our data also suggest a better clinical behavior for patients with thymomas accompanied by MTCs than patients with thymomas unaccompanied by those cysts, although further investigation is needed.