Radionuclide dacryocystography (RNDCG) is a safe, physiologic method of evaluating lacrimal drainage. The technique is performed by placing a radioactive "tear" in the eye(s) and following the passage of radioactivity through the lacrimal drainage apparatus with a scintillation (gamma) camera. RNDCG is indicated in any patient with tearing who has a lacrimal drainage system which is open to irrigation. It is also useful in children, when clinical signs are equivocal. Interpretation is made by comparing a patient's scan to "normal" scans, or in unilateral cases, by comparing the symptomatic side to the asymptomatic side. Many causes of functional or partial lacrimal drainage obstruction can be detected and differentiated from various tear hypersecretion abnormalities. RNDCG also has many research applications.