During a 4-year study period, January 1983 to December 1986, 24 patients (18 index cases) with spinal muscular atrophy (hereditary motor neuropathy, HMN), 9 with myasthenia gravis (MG), 6 with progressive supranuclear palsy (PSP), and 5 with subacute sclerosing panencephalitis (SSPE) were diagnosed in Benghazi. The HMN group comprised 6 acute infantile, 12 chronic childhood, and 3 each with adult-onset proximal, and distal forms of the disease. The crude average annual incidence of acute infantile HMN was 0.3/100,000 total population and 1/12,500 births in Benghazi. The crude prevalence rates of chronic childhood, adult-onset proximal, and distal types of HMN were 2.3, 0.6 and 0.6/100,000, respectively. The larger family size and the high rate of consanguineous marriages contribute to the high frequency of HMN in the study area. Distal HMN constituted 12.5% of the total cases. The adjusted average incidence of MG was 4.4/million/year, 2.1 for males and 6.8 for females. The female:male incidence ratio was 3.2:1. The crude average annual incidence rates/million inhabitants for PSP asnd SSPE were 3 and 2.4, respectively. The frequency of occurrence of SSPE among the subtropical Arab community under investigation is comparable with other surveys from the Middle East and Mediterranean region.