Biological markers of inflammation are useful for the diagnosis and monitoring of inflammatory rheumatic diseases. The present study tested, whether serum amyloid A (SAA) could be used as a marker of inflammatory disease activity in ankylosing spondylitis (AS). In 72 patients with AS, the two valuable surrogate markers of disease activity, erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP), and an established clinical activity score (Bath Ankylosing Spondylitis Disease Activity Index, BASDAI) were correlated to the serum levels of SAA. It was found that SAA correlates well with ESR, CRP, and BASDAI. Because of its strong correlation, SAA seems to be an additional very useful disease activity marker. When used in diagnosis, and especially in monitoring of inflammation, further studies are required. Another interesting point of view is the described role of plasma SAA as a precursor of Amyloid A (AA) protein in secondary amyloidosis, a known complication in AS. In all probability, high circulating SAA levels are a predisposing indicator of disease activity.