Uveitis is diagnosed by ophthalmologic and systemic examination, but it may be difficult to diagnose accurately in some chronic cases. We performed diagnostic and therapeutic vitrectomy for chronic uveitis in 6 eyes of 4 patients, and examined vitreous specimens with immunohistochemical staining. Visual acuity improved in all cases due to clearing up of the vitreous cells. One patient had sarcoidosis, one had B cell malignant lymphoma (ocular reticulum cell sarcoma), and two had reactive lymphoid hyperplasia. Vitrectomy for chronic uveitis is useful for diagnosis and improvement of vision, and we recommend immunohistological staining of vitreous specimens. Particularly, when intraocular lymphoma is suspected, vitrectomy must be performed early.