Assessment of tear drainage impairment after monocanalicular obstruction can be difficult. The authors used fluorescein dye disappearance to evaluate tear drainage after experimental obstruction of upper, lower, neither, or both canaliculi in 20 subjects. Marked impairment was noted in all subjects when both canaliculi were occluded (P less than 0.004). Monocanalicular obstruction (either upper or lower) generally resulted in minimal or no impairment, though 10% of subjects showed marked impairment. Fluorescein dye disappearance proved a reliable, rapid method for assessing tear drainage and detecting lacrimal obstruction.