Serum glucose, serum immunoreactive insulin and sedimentation rate (ESR) were measured in eighteen male patients with ankylosing spondylitis (AS) and seven male healthy controls. The findings were correlated with the presence or absence of inflammatory activity of the disease. Fourteen patients had active AS with ESR of 47.0 +/- 27.7 mm; they had increased insulin levels measured as area under curve (AUC) of a glucose tolerance test 107.4 +/- 44.1 cm2 vs controls 40.8 +/- 12.6 cm2 (p less than 0.03). In 4 patients with clinically inactive AS and with ESR of 17.0 +/- 4.0 mm the insulin levels as the AUC were 83.2 +/- 38.0 cm2 vs controls (p = ns). In the whole group there was a direct correlation between ESR and serum immunoreactive insulin levels (r = 0.47 p less than 0.05). Our study suggests that AS may be associated with hyperinsulinism, whose role in the physiopathogenesis of the disease remains unknown.