Interannual variability of the atmosphere of Mars is dominated by the occurrence, or absence, of planet-encircling dust storms. In this paper we use a recent review of Earth-based telescopic observations of Mars together with Viking orbiter and lander data to estimate the frequency of occurrence of planet-encircling dust storms over the past century and to test whether the period spanned by the Mariner 9 and Viking missions to Mars is representative of the decades prior to 1950. In doing so, we take into account the practical limitations imposed on Earth-based coverage of Mars by the orbital geometries of the two planets. Both spacecraft and Earth-based observations suggest that planet-encircling dust storms on Mars occur during a so-called ''dust storm season'' in southern spring and summer. Viking data demonstrate decidedly that planet-encircling dust storms could have occurred in the past on Mars without being detected from Earth during years in which Mars was far from Earth during the dust storm season and thus difficult to observe. However, the same historical record indicates that planet-encircling storms were absent during the dust storm seasons monitored during several favorable oppositions prior to 1956 and after 1986. Overall, the chance of a planet-encircling dust storm occurring in any arbitrary Mars year is estimated to be approximately one in three (18--55% at the 95% level of confidence), if such occurrence is random from year-to-year and yet restricted seasonally to southern spring and summer. ¿ American Geophysical Union 1993