The aim of this study was to evaluate the long-term results and the factors influencing the success in patients with nasolacrimal duct obstruction treated with intranasal endoscopic dacryocystorhinostomy (DCR) and silicone tube intubation (STI). We prospectively investigated 158 patients with lacrimal obstruction in two groups, one of which comprised 108 patients treated primarily with intranasal endoscopic DCR by experienced surgeons and the other comprised 50 patients who were operated on by inexperienced surgeons. In a mean follow-up time of 49 months the surgical success was 94.4% in experienced hands and 58.0% in inexperienced hands. The endoscopic examination of six patients with failure in the first group revealed granulation tissue around the tube in four, atonic sac in one and persistence of bone that was supposed to have been excised in the nasal cavity in one. There were 21 failures out of 50 patients in the second group: granulation tissue in 2 cases, fenestration to the nasolacrimal duct instead of the sac in 6 cases, synechia between the lateral nasal wall and the middle turbinate in 2 cases, bony spicles causing obstruction in 5 cases and fenestration anterior to the sac in 2 cases. In 4 cases no reasons were found for failure, but perhaps the small fenestration and failure to remove the medial half of the membranous sac wall was the reason. DCR and STI can be performed for primary treatment in lacrimal obstruction. There is a learning curve for the operation. False localization of the lacrimal sac, granulation tissue formation around the tubes, retained bony spicles, inadequate removal of the medial wall of the sac and the synechia between the lateral wall and the middle turbinate are the most common causes of failure.