Favorable results of composite resin sealant restorations in narrow occlusal cavities raise the question of whether extended cavity preparation is justified. Larger restorations are subject to increased polymerization shrinkage. Microleakage of extended and nonextended composite resin sealant restorations was tested in this study. Forty extracted caries-free third molars were allocated to four experimental groups. Occlusal cavity preparations were prepared, varying from narrow cavities restricted to the central fossa to preparations eradicating all fissures. Composite resin restorations were inserted with or without a base and with or without a sealant. Control restorations were inserted in nonetched preparations. Microleakage occurred when cavities were not etched and occassionally when a sealant was not applied. Conclusions indicate that a class I composite resin restoration extended beyond the central fossa can be free of microleakage if covered with a fissure sealant.