Reissner's membrane(RM) is a thin membrane which consists of two cell layers. A distension of RM is consistently observed in endolymphatic hydrops. A quantitative study of the cellularity of human RM, however, has not been undertaken in normal temporal bones and temporal bones with endolymphatic hydrops. To quantify the cellullarity of normal RM, morphometric measurements of human RM were performed in the basal turn of the cochlea under light microscopy, using 30 normal temporal bones from 15 subjects with various age groups. The number of both epithelial and mesothelial cells for the entire width of RM was 40.57(ranging from 31.33 to 48.33). The ratio of epithelial cells to mesothelial cells was about 3 : 1. The average width of RM was 0.77mm(ranging from 0.60mm to 0.93mm). The cellular density of RM(as the number of cells per mm of RM) was 52.21(ranging from 40.40 to 65.60). The cellularity of normal RM decreased with increasing age. The result indicates that an age-related process of cellular degeneration can occur in normal RM.