Purpose: To characterize anatomical variations of the nasolacrimal canal in patients with complex congenital nasolacrimal duct obstruction. Methods: Retrospective case series of 25 children with congenital nasolacrimal duct obstruction. All children had failed probings and were examined by computed tomography. An analysis of imaging data is presented. Results: Anomalous development of the nasolacrimal canal was confirmed on computed tomography. Two main types of malformations were observed: fundamental (20 patients) and special (5 patients). In the fundamental type, the upper portion of the nasolacrimal canal was relatively normal and became significantly stenotic, or even atretic, at the middle and terminal segments. In the special type, the upper portion of the nasolacrimal duct was malformed. Only the special type showed an association with systemic abnormalities. In patients with unilateral fundamental type malformation (11 patients), the transverse and anteroposterior diameters of the upper segment of the nasolacrimal canal on the affected side were significantly larger than that of the normal side (P = .000). The height of the nasolacrimal canal on the normal side was significantly larger than that of the affected side (P = .000). Conclusion: Computed tomography is useful for delineation of anatomical characteristics of the nasolacrimal canal and to diagnose nasolacrimal canal malformation. Malformed nasolacrimal canals are of two main types: fundamental and special. Hydrostatic pressure in the lacrimal duct appears to the underlying mechanism as enlargement of the nasolacrimal canal in the unilateral fundamental type.