A description is made of a long run forest fire danger index. The index is based on the principle that forest fires follow a self-organized critical behavior, which establishes that under a wide variety of circumstances, forest fires maintain an exponential relationship over various orders of magnitude between frequency of occurrence and the burned area. A description is made of the principle of self-organized critical behavior of forest fires and, based on the records of surface and number of forest fires per State in Mexico from 1970 to 2005, it is proved empirically that the total number of forest fires; occurred per year in each State maintains an exponential relationship with the burned area. This behavior is used to define a long run forest fire danger index. The analysis of the results shows that the fires occurring in each State exhibit frequency-size distributions which approximate a power distribution. The importance of this result is that it permits the calculation of long run forest fire danger indices for each entity, indices that can have a wide variety of applications in the planning and application of the activities of prevention and control of forest fires.