To determine the pathophysiology and to discuss possible solutions for tube obstruction since this is a frequent complication in glaucoma drainage device surgery.81 eyes with a glaucoma drainage device (GDD) fitted by our Department during the last twelve years (1991-2002) were retrospectively reviewed and eleven cases of tube obstruction were detected. The treatments included Nd-Yag laser, anterior vitrectomy, blood clot removal and intracamerular rTPA.The obstruction was proximal in ten cases and distal in one. Of the former, the obstruction was caused by a blood clot in six cases, by vitreous in three cases and by a fibrinous membrane in one case. The nature of the distal obstruction was not determined. One case of vitreous obstruction occurred after a posterior capsulotomy with Nd-Yag laser two years after positioning the GDD. Complete success was achieved in five cases, and partial success in three where antiglaucoma therapy had to be maintained postoperatively. Complete failure was found in the other three cases.The material which most frequently caused the tube obstruction was blood and although in some cases this may resolve spontaneously, clot removal with forceps or rTPA intracameral injection may be helpful. In vitreous obstruction, Nd-Yag laser may temporariril clear the tube, but extensive anterior vitrectomy is required to maintain long-term permeability. We suggest carrying out a small posterior capsulotomy in pseudophakic patients with a GDD.