Objective: To investigate the effect of using different resin-compositerestorations for restoring conservative mesio-occluso-distal (MOD)cavities on the incremental and accumulative cuspal deflection.Methods: Forty extracted, sound human maxillary second premolar teethwere subjected to standardized MOD cavity preparation. Two groups, of20 teeth, were restored with either Filtek__ Z250 (3M ESPE, USA) orFiltek__ P90 (3M ESPE, USA). Cuspal deflection was calculated bymeasuring the intercuspal distance between the indexed cusp tips beforerestoration and after every five minutes till 30 minutes using astereomicroscope connected to a digital camera and image analysissoftware to detect the cuspal deflection.Results: Comparing the two groups at each time interval, it was foundthat there was no significant difference immediately after curing and atfive, 15, 20 and 25 minutes. However, a significant difference wasrecorded for 10 and 30 minutes time. Filtek__ P90 showed significantlyless accumulative cuspal deflection than Filtek__ Z250.Conclusions: Changes in cuspal deflection of the tested materials overtime are not consistent and are not constantly lower for silorane-basedresin composite. After five minutes of curing, silorane-based resincomposite surpassed the methacrylate-based resin composite incontrolling the accumulative cuspal deflection.Clinical significance: The change in the organic matrix of the resincomposite by using silorane has a positive effect on controlling theaccumulative cuspal deflection. However, this was only true after 5minutes of curing.