OBJECTIVE The aim of this study was to understand the long-term outflow pathway mechanisms after deep sclerectomy - when collagen implant is resorbed - using ultrasound biomicroscopy (UBM). METHODS Forty-three eyes of 32 patients with medically uncontrolled open-angle glaucoma at least 1 year after deep sclerectomy were studied in an observational, non-randomised, consecutive case series. Postoperatively 15 eyes (35.7%) had goniopuncture with the Nd:YAG laser. Four eyes (9.5%) had postoperative subconjunctival injections of mitomycin C and two eyes (4.7%) had an injection of 5-fluorouracil, because of intraocular pressure (IOP) increase. Complete examination and UBM of the filtering site were performed 1-6 years after surgery. The following parameters were assessed: (1) Presence of a subconjunctival filtering bleb; (2) presence and volume of an intrascleral cavity; (3) presence of a suprachoroidal hypoechoic area. RESULTS Intraocular pressure decreased significantly from 28.1+/-2.5 mmHg preoperatively to 12.4+/-3.8 (range 7-25) mmHg at the time of UBM (at least 1 year after surgery). Forty eyes showed clinically a diffuse filtering bleb. UBM demonstrated a subconjunctival space in all eyes. In 39 eyes (92.8%) an intrascleral cavity was observed. The mean volume of this cavity was 1.8 (range 0.11-6.53) mm(3). In 19 eyes (45.2%) we observed a hypoechoic area in the suprachoroidal space. CONCLUSIONS UBM examination demonstrated several aqueous humour drainage pathways. A low-reflective diffuse subconjunctival space meant persistent filtration in all eyes. More than 1 year after surgery 92.8% of eyes had a remaining intrascleral cavity. In almost half of the patients an additional suprachoroidal outflow was observed, significantly correlated with a lower IOP.