This supplement presents the results of a collaborative project between workers from several European nations. The study was started in order to provide data describing the distribution of the sensory hair cells in the normal human cochlea and to allow the evaluation of age-related changes in the hair cell density. Fifty-three cochleas (including nine from fetuses) were preserved by perilymphatic perfusion with fixative shortly after death. In the non-fetal material the hearing was clinically normal prior to death. Some subjects had audiograms available and these also had to be normal for their age for inclusion of the cochlea in the study. Dissection of cochleas permitted surface preparation techniques to be used to count the hair cells and allow the hair cell density to be described as inner or outer hair cells per mm. The total length in mm of each cochlea was also measured (Length). The location of each hair cell density count was defined as distance in mm from the base (Distance), distance in mm from the apex (Length minus Distance), or as a proportion of the total length of the cochlea when the location of that count was measured as distance from the base (Distance/Length). The material was allocated to one of six age bands. For each age band and at each point in the cochlea for which data existed the average hair cell density and its standard deviation were calculated. This allowed average cytocochleograms to be drawn for both inner and outer hair cells. The proportional method of representation of location within the cochlea gave the best fit of the available data and the proportional cytocochleograms are presented, although the data for all three methods are included in tabular form. The average cytocochleograms indicate a progressive age-related loss of outer hair cells. This loss was exacerbated at both the apical and basal ends of the cochlea. The overall loss was less marked for the inner hair cell population but was accentuated at the base, like that of the inner hair cells, although not at the apex. With the proportional datasets, and taking fetal age as zero, a model of hair cell loss was developed. A simple linear equation in the form: Hair cell density(i) = (Age in years X Age Coefficient(i)) + Constant(i) gave the best fit at each proportional location (i) for both inner and outer hair. The age coefficients and constants are given in tables for inner and outer hair cells.(ABSTRACT TRUNCATED AT 400 WORDS)