Over an 18-month period, all incident cases of neurological disorders were ascertained prospectively in an unselected urban population based in 13 general practices in the London area by a General Practice Linkage Scheme with the National Hospital for Neurology and Neurosurgery. In three of these practices, the lifetime prevalence of neurological disorders was also assessed. A population of 100 230 patients registered with participating general practices was followed prospectively for the onset of neurological disorders. Multiple methods of case finding were used to maintain accuracy. The age- and sex-adjusted incidence rates of neurological disorders were calculated. The lifetime prevalence of neurological disorders was surveyed in 27 658 of the patients. The age- and sex-adjusted incidence rates were calculated for major neurological conditions. [These are expressed as rates per 100 000 persons per annum, with 95% confidence intervals (CI) in parentheses]. The commonest of these were first cerebrovascular events, 205 (CI: 183, 230); shingles, 140 (CI: 104, 184); diabetic polyneuropathy, 54 (CI: 33, 83); compressive neuropathies, 49 (CI: 39, 61); epilepsy, 46 (CI: 36, 60); Parkinson's disease, 19 (CI: 12, 27); peripheral neuropathies, 15 (CI: 9, 23); CNS infections, 12 (CI: 5, 13); post-herpetic neuralgia, 11 (CI: 6, 17); and major neurological injuries, 10 (CI: 4, 11). Lifetime prevalence rates are also reported (expressed as rate per 1000 persons with 95% CI). The most prevalent conditions were: completed stroke, 9 (CI: 8, 11); transient ischaemic attacks, 5 (CI: 4, 6); active epilepsy, 4 (CI: 4, 5); congenital neurological deficit, 3 (CI: 3, 4); Parkinson's disease, 2 (CI: 1, 3); multiple sclerosis, 2 (CI: 2, 3); diabetic polyneuropathy, 2 (CI: 1, 3); compressive mononeuropathies, 2 (CI: 2, 3); and sub-arachnoid haemorrhage, 1 (CI: 0.8, 2). Overall, the onset of 625 neurological disorders was observed per 100 000 population annually. Six percent of the population had at some time had a neurological disorder. This is the first study of the incidence and lifetime prevalence of neurological disorders in recent times; we found that these disorders give rise to significant morbidity in the community.