Extralobar sequestration of pulmonary tissues is relatively rare and seldom produces symptoms. An incidental finding is described of extralobar sequestration of the lung in a Nigerian child, presenting as an anterior mediastinal polycystic mass. The mass was attached to the right pulmonary artery by a fibrovascular pedicle which contained a small elastic artery, veins, and nerve bundles. Sequestrated lobes are usually situated in the posterior basal lobes of the lungs and may be associated with other congenital abnormalities, but the case reported here differs in these respects. Death was due to enterocolitis unrelated to the congenital abnormality.