Comparison of hospital admission causes for previously known (group A) and HIV-infected patients diagnosed during in-ward stay (group B), from 2009 to 2011. Retrospective evaluation of demographic, epidemiologic, clinical, immunologic, virologic and treatment parameters at time of admission. 1167 patients were admitted; of those 617 (52,9%) were HIV-infected: 92% HIV-1 and 8% HIV-2. 83% had previously known HIV infection and 15% were diagnosed during hospital stay (missing data in 2%). 66% were male, mean age was 46 years and 52% were Portuguese. The most frequent transmission routes were heterosexual exposure (36%) and iv drug use (29%). Mean length of hospital stay was 17 days (group A) and 28 days (group B) (p = 0,004). At admission, the mean TCD4+ count was 280 cells/mm3 in group A, and 132 cells/mm3 in group B (p<0,001). The majority of group B patients had clinical or immunological AIDS criteria at admission (84%) while group A presented a 71% rate for the same parameter (p=0,011). In group A, 52% of patients were on antiretroviral therapy but of those only 33% presented undetectable HIV plasma RNA, non-adherence being an important cause of therapeutic failure identified in 40% of cases. Respiratory infection was the principal cause of hospital admission in both groups (33% in group A vs. 35% in group B). The most prevalent nosological entities were community acquired pneumonia in group A (18,1% vs. 11,5%-p=0,118) and Pneumocystis jirovecii pneumonia in group B (4% vs. 18%-p<0,001). Mycobacterium tuberculosis was frequently identified as an agent of opportunistic infection (10% in group A vs. 24% in group B-p=<0,001). HCV coinfection was a comorbidity found in 37% in group A vs. 11% in group B (p<0,001). Other relevant comorbidities were psychiatric disturbances (16% vs. 3%-p=0,001) and neoplastic conditions (11% vs. 0%-p=0,001), mostly present in group A. Mortality rate was not significantly different between groups (10% group A vs. 11% group B) (p=0,773). This analysis evidenced that, a significant percentage of HIV patients diagnosed at admission were late presenters. Slightly a half of patients with previous known HIV infection were prescribed cARV and only a third presented undetectable HIV viral load. Non-adherence was a major concern in this population. Respiratory infections had a significant clinical impact in both groups, justifying the importance of vaccination prevention strategies in immunocompromised individuals