Swaziland and Lesotho are the countries with the highest HIV prevalence in the world. These countries have in common another distinguishing feature: during the past century they sent massive numbers of migrant workers into South African mines. This paper examines whether mining activities in a bordering country affect HIV infections. A job in the mines implies spending a long period away from the household of origin surrounded by an active sex industry. This creates potential incentives for multiple concurrent partnerships. Using Demographic and Health Surveys, the analysis shows that migrant miners aged 30-44 are 15 percentage points more likely to be HIV positive and having a migrant miner as a partner increases the probability of infection for women by 8 percentage points. The study also shows that miners are less likely to abstain and to use condoms and that female partners of miners are more likely to engage in extra-marital sex. We interpret these results as suggesting that miners’ migration into South Africa has increased the spread of HIV/AIDS in the countries of origin. Consistent with this interpretation, the associations between HIV infection and being a miner or a miner’s wife are not statistically significant in Zimbabwe, characterized by a local mining industry.