We quantify foreshock occurrence probabilities by applying the empirical technique of Jones (1985) to the western Nevada and eastern California earthquake catalog compiled by the University of Nevada, Reno, from 1934 through 1991. The foreshock occurrence rates depend heavily on the parameters used to remove aftershocks from the catalog. It is necessary to separate the Mammoth/Mono region from the rest of the catalog to determine the parameters that most effectively remove the aftershocks from the catalog. The probability that an M ≧ 3.0 earthquake will be followed by an earthquake of larger magnitude within 5 days and 10 km is 10% in the Mammoth/Mono region and 6% in the Nevada region, and seems to be independent of the magnitude of the proposed foreshock. The probability that an earthquake will be followed by another one at least one magnitude unit larger is 1 to 2% in each region. These probabilities imply that the occurrence of an earthquake M ≧ 4.0 increases the possibility of a damaging earthquake of M ≧ 5.0 by several orders of magnitude above the low background probability. Most mainshocks occur within a few hours after a possible foreshock, and the probability that a mainshock will still occur decreases logarithmically with time after the proposed foreshock. These foreshock properties are similar to those in southern California and in other parts of the world, with the exception that the Mammoth/Mono region, a volcanic area, exhibits more swarm-like behavior than does the southern California or Nevada region.