Jails may foster the spread of HIV, particularly among drug users. In 2006, male injection drug users (n = 499) detained in Tehran consented to HIV testing at intake and discharge. HIV prevalence at intake was 24.4%. Nine of those who were HIV negative at intake were positive at discharge (annualized incidence rate 16.8%), including one p24 antigen positive. Jails may be contributing to the rapid spread of HIV in Iran and should be major points for prevention interventions.