The occurrence and distribution of fibrous long-spacing fibers (FLS) of lymph nodes in normal and pathological conditions have been examined by electron microscopy. The FLS can be easily identified by the characteristic broad cross bands with a 90 to 120 nm period. In normal lymph nodes, the FLS were most often found in the stalk of the valve projecting into the lymphatic sinus and in the perivascular matrix, and less frequently in the wall of the vein, in the sinus wall and in the stroma of the medulla at the hilus. In pathological conditions, a large number of FLS appeared in the hyalinous matrix of lymphocyte depletion type of Hodgkin's disease. These fibers were also observed in reticular matrices of lymphosarcoma and mixed cellularity type of Hodgkin's disease as well as in granulation tissues. It was noted that FLS appeared to be formed in the regions where abundant reticular fibers were laid down. Compared with the ultrastructural and histological findings in various lesions of lymph nodes, the possible mechanism and significance of FLS were discussed.