OBJECTIVE: To determine if a case of HIV-infection in a patient (GP) with common variable immunodeficiency, and with no knownrisk factors for HIV-infection, could be due to horizontal nosocomial transmission. METHODS: For determination of time oftransmission stored serum-samples from GP were analysed for HIV RNA content. Patient records were used to identify patients, whohad received intravenous therapy on the same day as GP. Samples from GP and these possible source patients were identified andphylogenetic analyses of the env, gag and RT-encoding region of pol were performed. Furthermore, routines in conjunction withintravenous therapy were examined. RESULTS: We identified a patient (FDL) harbouring virus almost indistinguishable from thevirus isolated from GP. The pairwise nucleotide distance between the C2-V3-C3 region of the env and gag sequences from the twopatients were 1.9 and 0.9% respectively. In addition, GP harboured HIV RNA with a foscarnet resistance mutation further lendingsupport to virus from the foscarnet-treated FDL being the source of the infection. Interestingly, GP experienced increases inimmunoglobulin production after contracting the HIV-infection, and decreases after antiretroviral-induced viral suppression. Aclinical procedure which, under stressful conditions, could lead to breaches in infection control measures was identified. The source ofthe infection was most likely a contaminated multidose vial. CONCLUSION: Through epidemiological and phylogenetic analyses acase of horizontal nosocomial HIV-transmission was disclosed. Identification of multidose vials as possible vehicles for horizontalnosocomial transmission recently led to the recommendation of restriction of the use of multidose vials, a recommendation supportedby the present study. The study underlies the importance of a constant survey of infection control precautions.