Mira variables are very bright objects in the IR (~2µm) so that they can be seen to large distances and through heavy interstellar absorption. Provided that they have well defined luminosities they should therefore be ideal indicators of galactic and extragalactic distances. Luminosities are also essential for understanding the physical state and pulsational characteristics of these stars which may be at the final stage of stellar evolution before planetary nebula formation1-3. A period-luminosity relation has been established for galactic Miras4 but the intrinsic scatter in the relation is largely unknown. We report here periods and bolometric magnitudes for Miras in the Large Magellanic Cloud (LMC) which show a period-luminosity relation with very small scatter.