A 14-year-old boy developed acute transverse myelitis with severe abdominal pain, bladder dysfunction, weakness, and sensory loss of the lower extremities. Magnetic resonance imaging revealed a segmental expanded central edema affecting parts of the spinal cord, including the caudal medulla oblongata. Antibody response toMycoplasma pneumoniaewas negative in microparticle agglutination assays (1:40 in the acute serum and 1:160 in the convalescent serum) and complement fixation tests (1:20 and 1:10). However, analysis of acute-phase serum revealed a specific IgA and IgG response but no IgM response. Detection ofM. pneumoniaein the cerebrospinal fluid by nested polymerase chain reaction and in nasopharyngeal aspirate by culture confirmed anM. pneumoniaeinfection. Treatment with doxycycline (100 mg daily) was started on the second day after admission to the hospital and continued for 14 days; the patient recovered completely and was discharged 20 days after onset of the disease, with no signs of neurological deficits.