Background: Acquired nasolacrimal duct obstruction is a blockage of the lacrimal outflow system usually caused by a local non-specific inflammation of the lacrimal sac and the nasolacrimal duct. However, cases exist where the primary nasolacrimal system obstruction was caused by malignancies. Our aim was to investigate lacrimal sac pathologies in patients with acquired nasolacrimal duct obstruction and compare its clinical manifestations. Methods: The retrospective clinical study included 275 cases with acquired nasolacrimal duct obstruction who underwent external dacryocystorhinostomy and lacrimal sac biopsy. Cases were classified into tumor or non-specific pathology groups subdivided according to the level of inflammation. Histological and clinical data were analyzed.Results: Three tumors (1.1%) (adenoid cystic carcinoma, eccrine spiradenoma and small B cell lymphoma) were diagnosed. Chronic non-granulomatous inflammation was the most common histological finding, corresponding to 194 cases (70.5%). Other 81 (29.5%) were subacute, chronic forms of non-granulomatous inflammation, tumor and fibrosis cases. Epiphora with continuous purulent discharge was the most common clinical sign reported by 144 (52.4%) patients, two (0.7%) patients had a palpable mass near the medial canthal tendon, which were identified as an eccrine spiradenoma and small B cell lymphoma. There was no significant difference in the clinical symptoms, its’ duration or case history between the non-specific pathology and the tumor groups (p = 0.292). Conclusions: Chronic non-granulomatous inflammation of the lacrimal sac was the most frequent etiology of acquired nasolacrimal duct obstruction. There were no associations between the histological findings and clinical presentation. The authors recommend a lacrimal sac biopsy only in cases when a tumor is clinically suspected.