To study the role of copper in inflammatory rheumatic diseases, serum copper, serum ceruloplasmin concentration, erythrocyte sedimentation rate, and radio-copper studies were performed in 11 male patients with ankylosing spondylitis, in 12 female patients with rheumatoid arthritis and in 7 normal male subjects. The occurrence of elevated serum copper and serum ceruloplasmin levels can be confirmed in our study for patients with ankylosing spondylitis and rheumatoid arthritis when compared with normal controls. A significant correlation was found for these parameters and the inflammatory activity, characterized by the erythrocyte sedimentation rate. If groups with similar inflammatory activity are compared, higher ceruloplasmin concentrations are found in ankylosing spondylitis than in rheumatoid arthritis, the plasma incorporation of radiocopper also being higher in ankylosing spondylitis patients. Therefore, and because of comparable total serum copper concentrations, the non-ceruloplasmin bound copper level is found to be significantly higher in rheumatoid arthritis patients than in the group of ankylosing spondylitis patients. The significant correlation between erythrocyte sedimentation rate and the cumulative 120-hour urine excretion of radiocopper is in good agreement with the chemical finding of an elevated urinary copper excretion found by others, supporting the concept that the elevation of serum and urine copper levels in inflammatory rheumatoid diseases can be considered as an acute phase response.