ObjectiveThe association between HIV infection and the risk of venous thromboembolism (VTE) is controversial. We examined the risk of VTE in HIV-infected individuals compared with the general population and estimated the impact of low CD4 cell count, highly active antiretroviral therapy (HAART) and injecting drug use (IDU). MethodsWe identified 4333 Danish HIV-infected patients from the Danish HIV Cohort Study and a population-based age-and gender-matched comparison cohort of 43 330 individuals. VTE diagnoses were extracted from the Danish National Hospital Registry. Cumulative incidence curves were constructed for time to first VTE. Incidence rate ratios (IRRs) and impact of low CD4 cell count and HAART were estimated by Cox regression analyses. Analyses were stratified by IDU, adjusted for comorbidity and disaggregated by overall, provoked and unprovoked VTE. ResultsThe 5-year risk of VTE was 8.0% [95% confidence interval (CI) 5.78-10.74%] in IDU HIV-infected patients, 1.5% (95% CI 1.14-1.95%) in non-IDU HIV-infected patients and 0.3% (95% CI 0.29-0.41%) in the population comparison cohort. In non-IDU HIV-infected patients, adjusted IRRs for unprovoked and provoked VTE were 3.42 (95% CI 2.58-4.54) and 5.51 (95% CI 3.29-9.23), respectively, compared with the population comparison cohort. In IDU HIV-infected patients, the adjusted IRRs were 12.66 (95% CI 6.03-26.59) for unprovoked VTE and 9.38 (95% CI 1.61-54.50) for provoked VTE. Low CD4 cell count had a minor impact on these risk estimates, while HAART increased the overall risk (IRR 1.93; 95% CI 1.00-3.72). ConclusionHIV-infected patients are at increased risk of VTE, especially in the IDU population. HAART and possibly low CD4 cell count further increase the risk. DOI: 10.1111/j.1468-1293.00869.x HIV Medicine (2011 r 2010 British HIV Association 202 unrecognized even in modern health care systems [4]. It is important to clarify the main risk factors for VTE in order to identify individuals who may benefit from primary thromboprophylaxis [4,5]. Since the introduction of highly active antiretroviral therapy (HAART), HIV has become a chronic disease and life expectancy has increased substantially [6][7][8]. However, HIV-infected patients still experience considerable longterm treatment-associated morbidity.Recent studies of vascular disease in HIV-infected patients have focused on potential atherosclerotic complications in HAART-exposed patients [9,10]. In contrast, few studies have examined the risk of VTE in HIV-infected patients in the HAART era [11][12][13][14][15][16][17][18]. Studies mainly from the pre-HAART era estimated that the overall risk of VTE in patients with HIV infection was roughly 2-10-fold higher than expected in the general population [11][12][13][14][15][16][17][18][19][20][21][22][23][24]. However, many studies were limited by small sample size [15,16,22,23] and a lack of a population-based comparison cohort [11][12][13]15,16,19,[21][22][23].AIDS-related opportunistic infections, neoplasms and HIV infection per se have been hypothesized...