Complex sex-biased dispersal patterns often characterize social-group-living species and may ultimately drive patterns of cooperation and competition within and among groups. This study investigates whether observational data or genetic data alone can elucidate the potentially complex dispersal patterns of social-group-living black and white colobus monkeys (Colobus guereza, "guerezas"), or whether combining both data types provides novel insights. We employed long-term observation of eight neighbouring guereza groups in Kibale National Park, Uganda, as well as microsatellite genotyping of these and two other neighbouring groups. We created a statistical model to examine the observational data and used dyadic relatedness values within and among groups to analyse the genetic data. Analyses of observational and genetic data both supported the conclusion that males typically disperse from their natal groups and often transfer into nearby groups and probably beyond. Both data types also supported the conclusion that females are more philopatric than males but provided somewhat conflicting evidence about the extent of female philopatry. Observational data suggested that female dispersal is rare or nonexistent and transfers into neighbouring groups do not occur, but genetic data revealed numerous pairs of closely related adult females among neighbouring groups. Only by combining both data types were we able to understand the complexity of sex-biased dispersal patterns in guerezas and the processes that could explain our seemingly conflicting results. We suggest that the data are compatible with a scenario of group dissolution prior to the start of this study, followed by female transfers into different neighbouring groups.