Kidney biopsies from five patients with ankylosing spondylitis were studied electronmicro-scopically. In four patients renal arteriolar changes were observed. The structure and distribution of the lesions corresponded to the descriptions of renal arteriolar hyalinosis occurring in other conditions. There was nothing to suggest that the deposits originated from the cells, the elastic tissue or the basement membrane of the arteriolar wall. Their origin appears likely to be haematogenous. Immunological events are possibly involved in producing the vascular lesion.