The purpose of this study was to investigate the function of human cementum from a morphological viewpoint. To accomplish this purpose, the incremental lines in human acellular cementum were studied by light microscopy, microradiogram (CMR), scanning electron microscopy, and transmission electron microscopy, and the results were as follows: 1. In acellular cementum, many fine incremental lines of about 0.5 micron width were observed. 2. The fine incremental lines were observed running perpendicularly or obliquely to Sharpey's fibers in the acellular cementum. 3. The fine incremental lines observed in acellular cementum were not straight lines but had wave-like structure. 4. Each wave observed in the fine incremental lines corresponded to a bundle of Sharpey's fibers. 5. The fine incremental lines observed on Sharpey's fibers in acellular cementum showed specific characteristic features. 6. Despite the fine incremental lines showing a close relationship to the degree of calcification, they also seemed to show a strong relationship to the quality of the organic component of the acellular cementum. 7. Apart from the fine incremental lines which are specific to acellular cementum, incremental lines similar to those observed in cellular cementum were also observed within the acellular cementum. These incremental lines were wider and their degree of calcification was higher.