A 35-year-old man, with positive HBV and HCV markers, showed clinical and histopathological features of polyarteritis nodosa (PAN), in the course of HLA-B27 positive ankylosing spondylitis (AS). The possible occurrence of both diseases in genetically susceptible subjects is discussed. Although the role of HBV surface antigen in the pathogenesis of PAN is well established, there is still a large proportion of PAN patients with no evidence of HBV infection. In the present case, the coexistence of HCV infection led us to speculate about a possible role of this virus in the pathogenesis of PAN. In our case we were able to verify the sensitivity of single photon emission computed tomography (SPECT) in the diagnosis and the clinical evaluation of the ischaemic cerebral lesions.