We studied the incidence of monosymptomatic optic neuritis (MON) in Stockholm county, Sweden and its variation with person-related factors. Patients with suspected or diagnosed MON between January 1, 1990 and December 31, 1995 were referred from ophthalmologists and neurologists to a research registry. The diagnosis was based on accepted clinical criteria only, and verified by an ophthalmologist who examined all the patients. Data were collected by interview using a structured questionnaire. The crude mean annual incidence, based on 147 patients, 118 females and 29 males, diagnosed with MON, was 1.46 per 100,000 person-years, 2.28 for females and 0.59 for males. The corresponding age-adjusted incidences were 1.40, 2.28 and 0.53. The age-specific incidence curve for both sexes suggested a bimodal distribution with peaks at 30-34 years and 45-49 years. The smoothed cumulative incidences in 1 year birth cohorts showed a notchy profile, related to bimodality. The incidence among residents born out of the Nordic countries was low, 0.28 per 100,000. Patients with onset of MON before 40 years of age had a significantly higher frequency of mononuclear pleocytosis in cerebrospinal fluid and shorter duration to conversion to multiple sclerosis. In summary, MON occurred in Stockholm at a relatively low frequency, particularly among males. The presence of particular birth date and birth place related patterns might be etiologically relevant.