We have previously shown that individuals carrying the IL1RN CTA (TTG-0) haplotype had significantly lower synovial fluid levels of IL-10 and showed a trend towards lower levels of IL-1β and IL-6.13 We here report that in patients with both OA and RA, carriers of the TTG haplotype exhibit reduced plasma levels of IL-1Ra compared with CTA carriers. This evidence concerns the gene IL1RN and rheumatoid arthritis.