We evaluated the capability of ultrasound biomicroscopy (UBM) to predict fibrovascular proliferation at sclerotomy sites in eyes with postoperative vitreous hemor-rhage due to proliferative diabetic retinopathy (PDR). Ultrasound biomicroscopy was used for examining the sclerotomy sites in 13 eyes of 11 patients with PDR experiencing postoperative vitreous hemorrhage (PDR group). Thirty-nine sclerotomy sites (all entry sites of each eye) were examined before reoperation, and the UBM images were compared with findings obtained during revision of the vitrectomy. Thirteen eyes of 13 patients undergoing vitrectomy for nondiabetic dis-eases were used as controls and examined after vitrectomy. The UBM images were classified into the following four categories: A, tent; B, spheroid; C, trapezoid; and N, none. The findings were distributed as follows in the PDR group: category A, 18%; B, 5%; C, 56%; and N, 21 %; and as follows in the control group: category A, 28%; B, 5%; C, 5%; and N, 62%. In the PDR group, 11 of 12 sclerotomy sites disclosing fibrovascular proliferation possessed the trapezoidal image. Mean length of trapezoidal base was 2.49 ± 0.97 mm and 1.51 ± 0.75 mm in the groups with and without fibrovascular proliferation, respectively (P < 0.01). The average relative reflectivity of the trapezoidal image against the sclera was 0.501 ± 0.169 in the fibrovascular proliferation group and 0.891 ± 0.183 in the fibrous ingrowth group (P < 0.01). Ultrasound biomicroscopy is useful in detecting fibrovascular proliferation at sclerotomy sites because a large and low-reflecting trapezoidal UBM image is highly correlated to its presence.