A prospective clinical study (1974-1988) was carried out in 33 patients with several types of systemic vasculitis (SV) presenting as fever of unknown origin (FUO) according to the 1961 Petersdorf and Beeson's criteria. Histological confirmation, either from biopsy or necropsy, was available in all cases. The types of SV with FUO were: panarteritis nodosa (PAN) (14 cases), giant cell arteritis (GCA) (13 cases), and overlapping polyangiitic syndrome (OPS) (6 cases). In PAN, the clinical features associated with fever at the onset of the disease were remarkably nonspecific: constitutional symptoms (85%), arthromyalgia (50%), nonspecific abdominal pain (28%), and irritative cough (28%). In the whole course of the cases of GCA a significantly smaller frequency of presentation of local arterial symptoms and polymyalgia rheumatica (p less than 0.01) was found in the subgroup of patients with FUO than in those without it. As regard laboratory data, a higher increase of serum alkaline phosphatase (p less than 0.05) was found in the cases of PAN and OPS with FUO. Striated muscle biopsy and arteriography were the most useful investigations in the diagnosis of PAN. The study that gave the diagnosis in the cases of GCA was temporal artery biopsy.