The discovery of high levels of mercury in archaeological human bone in Portugal and Spain can be traced to using cinnabar as a pigment, preservative, or other uses that resulted in exposure to inorganic mercury.  This mercury entered the blood stream and was deposited in bone during growth and remodeling.  Over time, the exposure could have caused ill affect or even death.  The mercury in cinnabar also can be traced to its source using Hg stable isotopes.  Two archaeological bones from Portugal contain mercury with isotope ratios that match those in cinnabar from a mine at Almaden, Spain, the largest mercury mine and source of cinnabar in the world. This mine is known to have been used as early as the Neolithic in Spain and has been an important source for cinnabar pigment through Roman times. I review this prehistoric use of cinnabar with new data on additional sites that document mercury exposure in this region during the Megalithic period.