Although forest guenons and patas monkeys typically live in one-male, multi-female groups, during some breeding seasons several males enter heterosexual groups and compete for mating opportunities. The occurrence of these multi-male influxes makes it possible to compare female sexual behavior in the presence of one familiar versus multiple unfamiliar males, thus creating an ideal system for evaluating the paternity confusion hypothesis. In this study, I monitored female sexual behavior and collected fecal samples for the extraction of ovarian steroids from females in two habituated blue monkey groups (Cercopithecus mitis) over three breeding seasons. During this period, a multi-male influx occurred twice in each of the monkey groups, and a resident-male turnover took place in one group between breeding seasons. In support of the paternity confusion hypothesis, females were sexually active for periods well beyond the time surrounding ovulation, and they also mated during pregnancy and prior to the onset of ovarian cycling. Moreover, following the resident-male turnover, several females rapidly became sexually active even though they showed no sign of ovarian function. However, while females did have longer sexually active periods when multiple males were in their group, they were no more likely to mate during pregnancy or prior to the onset of ovarian cycling in influx as compared to non-influx seasons. Endocrine profiles compiled from fecal samples collected near the end of amenorrheic periods revealed that females consistently had high estrogen levels at this time, and estrogen levels are also known to rise at certain points during pregnancy. Potentially, the rise in estrogen levels at these times when females are not otherwise ready to conceive is part of an adaptive mechanism through which the likelihood of mating is increased, irrespective of the number or familiarity of males who are present. Alternatively, the increase in estrogen levels may be a side-effect of endocrine changes needed to maintain pregnancy and reactivate the reproductive system following amenorrheic periods.