Estimating the prevalence of accumulated HIV drug resistace in patients receiving antiretroviral therapy (ART) is difficult due to lack of resistace testing at all occasions of virological failure and in patients with undetectable viral load. A method to estimate this for 6498 EuroSIDA patients who were under follow-up on ART at 1 July 2008 was therefore developed by imputing data on patients with no prior resistance test results, based on the probability of detecting resistance in tested patients with similar profiles.