We will present the first two cases of HIV-1 group O in the UK. We will discuss the epidemiology, diagnostic difficulties, monitoring, and treatment of HIV-1 group 0. A 47 year old Caucasian man presented with a history of SOB, fever, and unproductive cough. His past medical history included malaria, an admission with PUO in 1998, and the investigation of a focal fit and persistent anaemia. Chest x ray demonstrated bilateral lung shadowing. ABG: pO2 = 6.93 kPa, pCO2 = 3.51 kPa, pH = 7.485. He was then referred to our team and it was noted he was lymphopaenic with a raised ESR. He was empirically treated for PCP with intravenous co-trimoxazole and steroids and improved. HIV antibody testing was initially reported as inconclusive, but suspicious of an HIV sub-type confirmed by the reference laboratory to be HIV-1 Group O. His CD4 count was 24 (5%) and viral load was <50 (Chiron bDNA). A sample sent to Madrid, however, gave a result of 74 132 copies/ml. Genotyping demonstrated resistance to NNRTIs and 2°mutations to protease inhibitors. He was started on HAART and had a significant CD4 rise to 207 (19%) four weeks into treatment. Unfortunately, he developed disseminated MAI exacerbated by immune reconstitution and his HAART was interrupted. He was started on MAI treatment and one month later his antiretroviral medication was reinstituted. The second case was his wife , a 37 year old Cameroonean woman whose baseline CD4 count was 170 (12%) and viral load 15 770 copies/ml. [ABSTRACT FROM AUTHOR]