OBJECTIVE To evaluate endoscopic and external dacryocystorhinostomy (DCR) using a quantitative, objective functional tool. METHODS This study comprised 58 cases divided into 4 groups: healthy controls; patients suffering from epiphora due to nasolacrimal duct (NLD) obstruction; patients who had undergone endoscopic DCR; and patients who had undergone external DCR. The lacrimal sac pressure was measured during blinking, forced blinking and the Valsalva maneuver using a polygraph. RESULTS Negative pressure was detected during blinking and forced blinking in all normal subjects and in most patients who had successfully undergone DCR. In contrast, positive pressure was detected in cases with epiphora and patients in whom DCR had failed. Negative pressure was higher after endoscopic than external DCR. During the Valsalva maneuver there were no pressure changes in normal cases and patients with epiphora. In contrast, positive pressure was detected after all of the successful procedures (being higher after external than endoscopic DCR) and in most of the patients in whom external DCR failed. CONCLUSIONS In normal subjects, negative pressure is created during blinking. In cases with epiphora due to NLD obstruction, the lacrimal pump is affected but its function is restored after successful DCR. The suction power of the pump mechanism is more effective after endoscopic than external DCR.