Sometimes in glaucomatous patients treated with trabeculectomy there is no correlation between bleb shape and intra-ocular pressure (IOP). In this study we evaluated the ultrasound biomicroscopy (UBM) features of filtering blebs after laser suture lysis (LSL) trabeculectomy in order to analyse whether its ultrasound-biomicroscopic image can predict the function (IOP). Methods: The Humphrey ultrasound biomicroscope, using a high-frequency (50-Mhz) probe, provides high-resolution images of filtering blebs. A total of 103 filtering blebs after LSL trabeculectomy were analysed by UBM. Taking into account the characteristics of internal reflectivity and scleral flap, we classified the blebs into three groups (good, fair, poor) that indicate the bleb function and correlated this UBM pattern with the IOP control (good, borderline, failure). Results: There was a statistically significant corelation between the UBM classification of function and the IOP control level. Both well-functioning and failed trabeculectomies could be identified by UBM. The UBM images of eyes with good IOP control are characterized by better visibility of the route under the scleral flap and a low reflectivity inside the bleb. Conclusions: In accordance with previous studies, we believe that UBM can be a useful method to study and explain the mechanisms of filtering structures and, together with IOP control, to evaluate the bleb function.