We found a high prevalence of HIV among injecting drug users (IDU) 54% in Tallinn and 90% in Kohtla Jarve, Estonia. Risk factors for HIV in Tallinn included use of the drug 'china white', being registered as an IDU at a drug treatment clinic, and sharing injecting equipment with sex partners. Differences existed in risk behaviour between the cities. An urgent scale-up of HIV prevention is needed. It is also important to explore how local 'risk environments' mediate the risk of HIV transmission.

A continuing HIV epidemic among new injectors is of critical public health concern. Interventions to prevent initiation into injecting drug use and scaling up HIV prevention programs for IDUs in Estonia are of utmost importance.

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Few recent publications have highlighted theoretical and methodological challenges using respondent-driven sampling (RDS). To explore why recruitment with RDS may work in some populations and not in others, we assess the implementation of RDS to recruit female sex workers (FSWs) and injection drug users (IDUs) into a human immunodeficiency virus biological and risk behavior survey in Tallinn, Estonia. Recruitment of FSWs was slower and more challenging than that of IDUs. The IDU study recruited 350 participants within 7 weeks, while the FSW study recruited 227 participants over 28 weeks. Implementation modifications that did not negatively impact key RDS theoretical and methodological requirements were used to improve recruitment during the FSW study. We recommend that all RDS studies include a formative research process to involve the participation of target populations and key persons associated with these populations in the study planning and throughout the implementation processes to improve recruitment from the outset and to respond to poor recruitment during data collection.