The purpose of this study was to evaluated the microleakage around preventive composite fillings used in occlusal fissures with incipient caries, and to assess the influence of the consistency of the composite resin upon the leakage. The fissure enamel of 30 extracted maxillary premolars were partially removed using a small round bur (Ash No. 1/2). Dentin was not exposed. The cavities were etched with phosphoric acid and filled with undiluted composite (Adaptic) or composite diluted with different amounts of liquid resin (Adaptic Bonding Agent). Five teeth were not etched before filling, and served as a control group. The teeth were cycled thermally between two dye solutions and the marginal penetration of dye was evaluated by examining sections of the crowns in a dissection microscope. Marginal leakage occurred in all the control teeth and in none of the acid-etched teeth. It may be concluded that the preventive composite filling therapy provides good sealing of pits and fissures with incipient caries, and that the consistency of the composite used is not of primary importance.