We report two patients with idiopathic acute lumbosacral myelitis, a rare form of acute transverse myelitis. Both patients developed urinary retention, moderate motor and sensory paresis of the lower extremities, severe sensory deficit in the anogenital region and reduced deep tendon reflexes. Steroid pulse therapy was initiated within 2 days after onset, and progress of the symptoms stopped immediately after administration in both of our patients. The sequelae of the sensory deficits in the sacral dermatome distribution and urinary retention impaired daily functioning. Immediate immunosuppressive therapy, including high-dose steroid treatment, is important to improve the prognosis of acute lumbosacral transverse myelitis.