Herpes simplex virus (HSV) myelitis has previously been reported to be a form of acute ascending necrotizing myelitis caused by HSV type 2 (HSV-2). We studied neurological symptoms, clinical course, magnetic resonance imaging (MRI) findings, and diagnosis by polymerase chain reaction (PCR) methods in 9 patients with HSV myelitis. In 6 cases, disease onset was marked by sensorimotor disturbances of lower extremities and urinary disturbances, with the transverse myelopathy gradually ascending to the cervicothoracic spinal cord level. The other 3 cases showed transverse myelopathy without an ascending pattern. Six cases showed acute progression, while 3 cases showed a subacute course. There were 2 cases with recurrent episodes. Three patients recovered, however, in the remaining 6 patients severe sequelae such as paraplegia persisted despite antiviral therapy. MRI showed a hyperintense lesion on T2-weighted images. Gadolinium enhancement was observed in 2 cases, and 1 case showed a hyperintense lesion both on T1- and on T2-weighted images, suggesting hemorrhagic necrosis. HSV-2 was detected by PCR techniques in all 6 cases with an ascending pattern. HSV-1 DNA was detected in 2 and HSV-2 DNA in 1 of the 3 cases with a nonascending pattern. Our findings demonstrate diverse clinical manifestations of HSV myelitis.