From the introduction of vitrectomy, infectious endophthalmitis was one of the indications for this surgical technique. Vitrectomy was later found to be a valuable method in both diagnostic and therapeutic evaluation of chronic uveitis.Twenty-eight eyes of 25 patients were operated on with purely diagnostic intent. These were patients not responding, or no longer responding, to their cortisone treatment and for whom no etiology had been found previously.Vitrectomy itself yielded the diagnosis in nine eyes of eight patients. Three had unexpected infectious pathology (one bacterial, one mycotic, one viral), while five had tumoral pathology four with non-Hodgkin lymphoma and one with a metastasis of a malignant melanoma of the skin. In four other patients, typical fundus lesions were seen during the surgery, providing a clue to the etiology: three cases of retinal necrosis and one case of a pseudotumoral mass suggestive of Toxocara canis. Serologic tests confirmed three cases of herpes simplex infection and one of Toxocara canis. Apart from the fact that a diagnosis could be made in half of the patients, the diagnostic vitrectomy also had a favorable effect upon vision in half the cases.In cases of chronic uveitis where no etiology has been found, vitrectomy is able to provide a diagnosis in about one-third of eyes directly. During surgery a typical appearance of the fundus may reveal a supplementary diagnosis, resulting in an overall diagnosis in about half of the cases. Moreover, half of the patients will have improved vision after surgery.