9-Hydroxy-10,12-octadecadienoic acid (9-HODE) and 13-hydroxy-9,11-octadecadienoic acid (13-HODE) are accumulated in the low density lipoproteins of patients suffering from rheumatoid arthritis for a factor of 20-50 compared to healthy individuals of the same age. Both acids, derived by lipid peroxidation of linoleic acid, induce the release of interleukin 1 beta. The latter induces bone degression. The genesis of 9- and 13-HODE seems therefore to be an important factor in the development and progression of rheuma; in addition 9-HODE was reported to be a stimulus of inflammation, comparable to leukotrienes.