In patients with established ankylosing spondylitis (AS) and a healthy control group, plasma levels of IGF-1 and TNF-alpha as well as possible connections with biochemical markers of the bone metabolism, humoral inflammatory activity (ESR, CRP), clinical manifestations, and an established clinical activity score (Bath Ankylosing Spondylitis Activity Index = BASDAI) were examined. In AS-patients (men and women) significantly increased TNF-alpha levels were found. Moreover, patients with enthesopathy showed a significantly more frequent increase of CRP and TNF-alpha levels besides an increased urinary pyridinium cross-link excretion. In addition, a significant positive correlation between TNF-alpha, CRP, BASDAI, and urinary pyridinium cross-link excretion was proved, besides a significant negative correlation of IGF-I to urinary pyridinium cross-links and TNF-alpha levels. Summing up, it may be said that TNF-alpha seems to be a reliable surrogate marker in enthesitis. This was proved so far for IgA and endothelium stimulating angiogenic factor only. Besides, the present results argue against a stimulation of osteogenesis. The catabolic situation under high TNF-alpha and low IGF-1 levels may play an important role in the pathogenesis of osteoporosis in AS.