We offer a new hypothesis for why HIV infections fell rapidly after 2001 in Russia: the Taliban's opium eradication campaign in Afghanistan reduced the supply of heroin, causing use to fall and, thus, transmission of HIV to fall. We present evidence of the impact of the eradication campaign on the heroin market and show that the fall in HIV infections happened simultaneously in Russia and surrounding countries soon after the eradication campaign. We also show that the decline in HIV infections only occurred in injecting drug users, while other risk groups were unaffected. Limitations to our analysis are discussed.