L'Hoest's monkeys have a short, dark brown coat, with a chestnut color across the back and a dark belly. Their cheeks are light gray with a pale moustache and they have a characteristic and prominent white bib. Their tail is long and hook-shaped at the end.
L'Hoest's monkeys are found in the upper eastern Congo basin. They occur in northeastern Democratic Republic of the Congo, Rwanda, Burundi, and western Uganda. These monkeys live in the moist and high primary forests. They will occupy a range of different kinds of forested areas, including gallery forest, mature lowland rain forests, wooded savanna at mountain slopes, and forest borders. However, they also will live on cultivated lands.
L'Hoest's monkeys live in fairly small groups dominated by females and have only a single male. The females are usually related, while the male stays only a couple of weeks or at most a couple of years. These monkeys are active during the day, mostly during early morning and late afternoon. They are mostly terrestrial but will forage and sleep in trees in a sitting position, usually either holding branches or each other. When they are alarmed or see they are being observed they will flee and take shelter in trees, and after become very still. Adult males make very loud and distinct calls. L'Hoest's monkeys also use different behaviors as a threat display. For example, staring includes fixing the eyes on a subject, raising the eyebrows, stretching the facial skin, and moving the ears back. They also often open their mouth without showing their teeth. Another threat display is head-bobbing, which icludes staring with an open mouth.
L'Hoest's monkeys are polygynous, which means that a single male mates with many females. These monkeys breed seasonally, with the timing depending on the area. After about a five-month gestation period, a single young will be born. Birth usually occurs at the end of the dry season, which allows lactation when rainfall is highest. When the infant is born the other females in the group will show much interest in the newborn and will try to hold it. After a few months nursing becomes less frequent, but will continue for about two years when the female gives birth to another infant. When young males reach reproductive maturity they will leave the group.
L'Hoest's monkeys suffer from the deforestation of their habitat, primarily due to agricultural expansion. These animals are also hunted for meat in some parts of their range; they are particularly vulnerable to both snaring and shotgun hunting.
The IUCN Red List and other sources don’t provide the number of the L'Hoest's monkey total population size. Currently, this species is classified as Vulnerable (VU) on the IUCN Red List and its numbers today are decreasing.
L’Hoest’s monkeys eat fruits and thus play an important role in seed dispersal.