A 58-year-old woman, diagnosed with primary obstruction of the nasolacrimal system, underwent external dacryocystorhinostomy and silicone stent implantation. During surgery, an incisional biopsy was obtained from the posterior wall of the lacrimal sac. Pathological examination revealed basal cell carcinoma of the lacrimal sac. Two weeks later, the diseased sac and surrounding tissue were excised, using frozen sections to ensure adequate tumor-free margins. The lacrimal sac tumor was diagnosed as basal cell carcinoma with focal squamous differentiation. No tumor recurrence was detected in 1.5 years of follow-up. The authors recommend that lacrimal sac biopsy should be performed in patients with an enlarged lacrimal sac and nasolacrimal system obstruction.