From March 1997 to June 1998, infectious etiologies of prolonged fever was prospectively investigated in 104 advanced human immunodeficiency virus (HIV) infected patients admitted to Siriraj Hospital. The etiology could be identified in 91 cases (87.5%). Of these, blood cultures from 68 patients yielded mycobacteria and fungi. Mycobacterium avium complex was the most common blood isolate in 24 per cent of the patients; followed by Mycobacterium tuberculosis in 20.2 per cent, Cryptococcus neoformans in 5.8 per cent, Penicillium marneffei in 5.8 per cent. During the course of febrile illness, 79 of the 91 patients (86.8%) exhibited focal lesions. Weight loss, elevated serum alkaline phosphatase were often found to be significantly more associated with MAC bacteremia (P < 0.05). Pulmonary involvement significantly correlated more with M. tuberculosis bacteremia than MAC bacteremia (P < 0.05). No cause could be identified in 13 cases. Mycobacterium blood culture alone established the etiologies in 68 cases (65.4%). Of the 25 patients with disseminated MAC (DMAC) infection, nine patients died during hospitalization. Another three cases died within a few months of appropriate anti-MAC chemotherapy. We concluded that the risk of DMAC infection in advanced AIDS patients in Thailand is high when low CD4 lymphocyte count is established. The prolonged fever resulted from DMAC in advanced HIV infection is warrant to be public health concern. Mycobacterium blood culture is a most valuable tool contributing to the diagnosis of infectious agents in this condition. The guidelines of 1997 USPHS/IDSA should be followed to give chemoprophylaxis against DMAC disease in patients with advanced HIV infection and a CD4 count less than 50 cells/mm3.