Serial clinical and electrophysiological examinations and immunological investigations have been performed on 34 patients with primary optic neuritis. In 8 patients, the latency of visual evoked responses was normal after the resolution of clinical symptoms, and in a further 5 patients improvement in VER latency occurred although the response remained abnormal. In 17 patients, VER's abnormal and did not alter over the period of clinical recovery; in 4 patients there was a progressive increase in the latency of the VER's through both eyes although a satisfactory clinical recovery occurred. The frequency of histocompatibility antigens was similar in the group of ON patients to that in the normal population. Immunological function was normal in patients with ON as assessed by levels of immunoglobulins. T and B lymphocyte numbers and by lymphocyte blastogenesis in response to PHA and myelin basic protein. Three patients have developed multiple sclerosis, all of whom had either HLA-A3 or -B7 antigens; the clinical and electrophysiological pattern of recovery was different in each case.