This study aimed to evaluate the semicarbazide-sensitive amine oxidase (SSAO) activity in human arterial tissues and in serum of patients with type 2 diabetes. The SSAO activity, with (14)C-benzylamine as substrate, was measured in homogenates of human inferior mesenteric arteries obtained at surgery, from 10 patients with type 2 diabetes and 16 non-diabetic patients and in the serum of 39 patients with type 2 diabetes and 40 non-diabetic control patients. The SSAO activity in the homogenates of vascular tissue was significantly lower in the diabetics than in the non-diabetics (P = 0.001). The SSAO activity in the serum of patients with type 2 diabetes was higher when compared with control group (P = 0.0001). In conclusion, the SSAO activity increased in the serum and decreased in the arterial tissue. These findings suggest damage in the vascular tissue and support the hypothesis that serum SSAO may be a useful biochemical marker for diabetes.