Pneumatocele of the lacrimal sac is an uncommon entity and has been rarely reported in literature. The valvular function of the Hasner's valve at the lower end of nasolacrimal duct is presumed to prevent the retrograde reflux of air into the lacrimal sac with increased intranasal pressure as in valsalva maneuver. Loss of this valvular function as noted with persistent positive airway pressure ventilation can cause retrograde air reflux into lacrimal sac. The authors report a case of lacrimal sac pneumatocele which developed following blunt nasal trauma involving the bony nasolacrimal duct.