Correspondence: SIR, We read with interest the recent paper by Sanders et al on the correlation of immunoglobulin and C reactive protein (CRP) levels in 22 patients with ankylosing spondylitis (AS) and 20 patients with rheumatoid arthritis (RA).' The authors found that IgA serum levels, though raised in AS, do not correlate with CRP levels as they do in RA, suggesting that the mechanism of increase of IgA in the two diseases may be different. They conclude that production of IgA in AS is unrelated to the stimulation of acute phase reactants, reflecting a specific mucosal immune stimulation, possibly in the gut. Thus IgA may be a marker of the pathogenesis of AS.