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Background: Information about changes in the trabeculectomy ostium and adjacent structures related to flattening of the anterior chamber in the early postoperative period may be important for management. We assessed the ostium and adjacent structures by means of ultrasound biomicroscopy (UBM) in order to identify common patterns related to postoperative flattening of the anterior chamber., Methods: We studied 15 eyes of 15 consecutive patients. A first UBM examination was performed when the patient was referred. Changes in the ostium (evidence of tissue adhesions to the ostium) were evaluated with UBM after the chamber flattening resolved, at least 60 days after the first examination. The following features were noted: depth of the anterior chamber, size and patency of the ostium, rotation of ciliary processes, presence of uveal effusion and ultrasound functioning of the bleb., Results: All the patients had ring-shaped ciliochoroidal effusions; in nine eyes the effusions were found only with UBM. In four eyes an anterior-posterior deformation of the ostium was found. Synechiae in the ostium were identified in the three eyes in which iris remnants had been recognized at the site on the initial UBM examination., Interpretation: Our observations confirm the presumed pathophysiology of flat anterior chamber without bleb leak. Ring-shaped effusions were detected on UBM even in cases in which conventional ultrasonography showed no positive results. UBM is a helpful tool in the diagnosis and management of flattening of the anterior chamber after trabeculectomy.