Patas monkeys may be especially vulnerable to local extinction because they live in relatively small, female-philopatric groups at low densities and are strongly polygynous. We assessed a patas monkey population in Kenya's 9,700 km2 Laikipia District over 25 years, using data collected in 1979–1981 and 1992–2004. The data were based on intensive observations of three study groups, “on the ground” counts, and surveys of Laikipia residents. In 1979–1981, a minimum of 415 patas monkeys lived in 14–15 groups. By 2000, the best estimate suggested 310–445 patas monkeys living in 13–17 groups over a greater surveyed area, suggesting that patas monkeys in Laikipia may have undergone a slight decline in numbers over time. Their distribution, however, was similar over time. The relative stability of this population has likely been the result of beneficial co-existence with large-scale cattle ranching. Outside Laikipia, substantial habitat alteration from rising human populations has coincided with the near disappearance of patas monkeys where they were previously more numerous. The small population in Laikipia, probably the largest remaining in Kenya, may therefore be critical to the continued existence of patas monkeys in that country and may be dependent on maintenance of large-scale ranches. Such land use provides patas monkeys with water and broad expanses of Acacia drepanolobium woodlands, the habitat to which patas are restricted in Laikipia. Am. J. Primatol. 69:1223–1235, 2007. © 2007 Wiley-Liss, Inc.