This report describes on a comparatively large reinforced concrete office building the result of long-term investigations into the sagging damage to its floor systems while it had been in service for about 25 years though repaired and extended after the first 1/4 roughly of that period. The five-storeyed construction framed with girders and columns, soon after its execution, started to develop floor deflections which had gradually increased until about six years after when a large part of the slab panels had been cracked and subsided by considerable amounts. To have the structure repaired so as to be restored to an assuredly stable state, the load-bearing capacity of one typically damaged slab was checked by a water-pressure loading test with the result that the other cases were also judged to reserve sufficient strengh safely to sustain maximum amounts of service loads then prescribed, and thus the subsided slab surfaces were levelled off with epoxy resin mortar and where practically required H-beams were fixed to girders across such slab panels to brace them. The eventual conditions of the floors of the repaired building, with a subsequent extension built thereon having slab-beam-girder systems, investigated just before its recent demolition, are provided in available detail. Also reference is made to primary causes of such slab injuries : they are most probably be ascribed to the want of slab thickness designed in the absence of pertinent code provisions and also to the construction process involving significant degrees of dimensional inaccuracies.