PURPOSE: To describe a case of persistent hypotony after an otherwise successful vitreous surgery for epiretinal membrane, in which ciliochoroidal detachment was detected by ultrasound biomicroscopy but not by ophthalmoscopy. METHOD: We used ultrasound biomicroscopy to determine the cause of persistent postoperative hypotony. RESULTS: Ultrasound biomicroscopy clearly disclosed ciliochoroidal detachment. In an attempt to resolve ciliochoroidal detachment, we performed vitrectomy and fluid-gas exchange combined with diathermy around the sites of sclerotomy. Postoperativeiy, the regression of ciliochoroidal detachment was confirmed by ultrasound biomicroscopy. CONCLUSIONS: Using ultrasound biomicroscopy, we were able to visualize ciliochoroidal detachment and its regression clearly, which we had not observed during ophthalmoscopy.