Terminal myelocystocele is a rare form of occult spinal dysraphism in which the hydromyelic caudal spinal cord and the subarachnoid space are hemiated through a posterior spina bifida. A 1.5 month old boy presented with a large lumbosacral mass and urinary incontinence. The magnetic resonance imaging, operative findings and pathological findings revealed a low lying conus with a dilated central canal dorsally attached to the subcutaneous tissue. Ventral subarachnoid space was enlarged and herniated through the laminar defect of the sacrum. The lesion was typical of a terminal myelocystocele. The clinical features are different from those of myelomeningocele in many aspects. Though the incidence is low, terminal myelocystocele should be included in the differential diagnosis of congenital lesions presenting as a lumbosacral mass.