From the study of our cases we think that the process does not seem to be a coincidence of both diseases but rather of the action of the X factor or factors of RA upon the domain of AS which would result in a variant of associate AS. This would be the true 'rheumatoid spondyloarthritis'. Its rate of frequency, since the predominance of AS marked by HL-A 27 is 6% in our country, would be somewhat higher than that reported by Fallet, if our hypothesis proves to be correct. Evidently it would not reach the ratio of 1/1600 which is the ratio to be expected if every factor of RA, by incidence of the HL-A 27, would cause this mixed picture.