Abstract: A process is described for the hydrocracking of heavy hydrocarbon oils which permits pitch conversions of over 90%, and preferably over 95%. A slurry of a heavy hydrocarbon oil and carbonaceous additive particles, such as coal, is passed in the presence of hydrogen through a confined vertical hydrocracking zone at high temperatures and pressures. An effluent which is almost entirely a gaseous phase is removed from the top of the hydrocracking zone, while a drag stream is removed from the remaining liquid in the hydrocracking zone. The top effluent has the advantage of being substantially free of pitch and metals, with the unreacted carbonaceous additives, metals and any unconverted pitch being all concentrated in the drag stream.