This work concerns the study of sediment formation in ebullated-bed hydroconversion of vacuum residues and investigates ways to avoid their formation at high conversion operation. Tests were carried out in a bench-scale unit at high temperature using a vacuum residue feedstock. Feeds and effluents were characterized to follow sediment formation, as well as conversion, yield structure, and hydrotreating performances. It was found that co-processing a vacuum residue feedstock with low amounts (<15 wt %) of heavy cycle oil from a fluidized catalytic cracking (FCC) unit can significantly improve the stability of the unconverted effluents. A detailed characterization of feedstock and products was carried out to understand the stabilization mechanisms. The effect of heavy cycle oil (HCO) on sediment reduction was attributed to the polycondensed tri-, tetra-, and pentaaromatics. It was also found that these species do not affect the hydroconversion performances and are refractory to conversion.