The purpose of this study was to evaluate the microleakage around preventive composite fillings in teeth subjected to mechanical loading and thermal cycling, and to assess the influence of the consistency of the composite resin upon the leakage. The fissure enamel was partially removed without dentin exposure in 30 extracted maxillary pre-molars. The cavities were etched with phosphoric acid and filled with undiluted composite (Adaptic) or composite diluted with varying amounts of liquid resin (Adaptic Bonding Agent). Five teeth were not acid etched and served as a control group. All teeth were mechanically loaded and unloaded, simulating masticatory forces, and subsequently thermally cycled between two dye solutions. Extensive leakage was seen around all fillings in the control group, while slight leakage, restricted to the feather-edge margin, was seen in five teeth with acid etched cavities. Three of these contained undiluted composite fillings. Twenty teeth showed no leakage. The results show that preventive composite filling provides good sealing of occlusal fissures, and that the seal is resistant to those thermal and mechanical stresses which may be expected in vivo. The use of diluted composite resin in preventive composite fillings is recommended.