The presence of cholesterol crystals has been suggested to be a factor interfering with periapical healing after conventional endodontic treatment. This investigation addresses the role of cholesterol crystals in impairing healing by studying the tissue response to the crystals, which were implanted in animals. Pure cholesterol crystals, prepared to a mushy form, were placed in Teflon cages that were implanted subcutaneously in guinea pigs. The cage-contents were retrieved after 2, 4 and 32 wk of implantation and processed for light and electron microscopy. The cages revealed delicate connective tissue that grew in through perforations on the cage-wall. The crystals were densely surrounded by numerous macrophages and multinucleated giant cells, forming a well-circumscribed area of tissue reaction. The cells, however, were unable to eliminate the crystals during an observation period of 8 months. The congregation of macrophages and giant cells, known to be major sources of apical inflammatory and bone resorptive mediators, suggest that accumulation of cholesterol crystals can be a factor in the failure of certain apical periodontitis lesions to resolve after conventional root-filling therapy.