Neuromyelitis optica (Devic's syndrome) typically occurs in a sporadic pattern. We studied the disorder in two sisters, ages 6 and 10 years, who presented with sudden loss of vision at age 3 years, followed 5 months later by a transverse myelopathy. The older girl had only light perception in either eye, persistent paraparesis and progressive scoliosis; her sister had a visual acuity of 20/200 in both eyes and mild spinal cord dysfunction. Results of electrodiagnostic tests correlated well with the severity of clinical neurologic deficits and provided objective estimates of the extent of tissue damage in both the optic nerves and the spinal cord. The presence of Devic's syndrome in these two sisters could be coincidental; however, the possibility of a shared genetic factor(s) should be considered.