A masonry wall structure comprising hollow concrete block units laid in courses, wherein each of the block units comprises front and rear panel members and a pair of side panel assemblies which form a gap between adjacent two of the block units. The side panel assemblies are connected to the front and rear panel members by means of reinforcing bars projecting rearwardly and forwardly from the front and rear panel members, respectively, into the gaps on both sides of each block unit, connecting bars projecting forwardly and rearwardly from the side panel assemblies into the gaps, steel wires binding overlapping end portions of the reinforcing and connecting bars together, and a body of cementitious material such as mortar filling the gaps and having completely buried therein those end portions of the bars which are bound together by the steel wires.