Most studies predicting the effects of prayer on health have examined intercessory prayer (prayers by others who often don't know you), yet most people pray for their own health and the health of others who they know. Our study, conducted in Miami, USA, differentiated praying for self, known others, and unknown others in people living with HIV, a virus with clearly defined biological markers of progression, enabling control for initial CD4-count and viral load. Only praying for known others predicted greater survival. People with HIV who prayed for known others were twice as likely to survive over 17 years compared to those who did not.