There is a lack of reliable information on the extent of the poliomyelitis problem in developing countries, although the disease is thought to be more of a threat in urban than in rural areas. The Expanded Programme on Immunization (EPI) began operations in Yemen in 1977, and it was considered appropriate to try to establish the prevalence of residual paralysis due to poliomyelitis in children aged 5-13 years, in order to estimate the annual incidence of clinical cases of the disease, and to determine the epidemiological features of poliomyelitis in the country. The data thus obtained would provide a basis for assessing the impact of the immunization programme on the incidence of poliomyelitis.The results of the survey showed a prevalence of lameness due to poliomyelitis of 4.0 per 1000 children. The estimated annual incidence of the disease is thus 18.6 per 100 000 of the general population, or approximately 1088 cases each year, with an estimated 163 deaths. An estimated 5000 children aged 5-13 years are lame as a result of poliomyelitis. There was no significant difference in the incidence of the disease in rural and urban areas. The median age of onset was 1.92 years in the urban setting and 1.29 years in the rural setting, with more than half of all cases occurring before the age of 2 years. Immunization efforts should therefore be directed towards infants aged under 2 years. Although a national disease notification system was established in 1976, 95% of the clinical cases discovered during the survey had not been reported. This underlines the importance of special surveys in gathering the data necessary to evaluate the effectiveness of the immunization programme.