Our knowledge of the progression and aetiology of hearing impairments is mainly inferred from cross-sectional studies of populations or individual case studies of relatively rare conditions. Longitudinal studies of carefully stratified samples enable scientific analysis of these two aspects of the ageing auditory system and also provide much needed incidence data on the basis of which to plan comprehensive hearing services. This preliminary paper using data from two studies over relatively short periods (between 2-4.5 years in Great Britain (GB) and up to 8 years in Denmark (DK] confirms (a) that deterioration of hearing impairment appears to be continuous and gradual for the majority (up to 97% on a 2-year assessment) with a median of about 5-6 dB/decade, and (b) that for mid-frequency average hearing levels applied to the samples of average age 55 (range 40-65) the incidence of hearing impairment is predicted accurately by interpolation of the relevant prevalence figures, and runs at about 1.8% per annum for 25+ dBHL bilateral hearing impairments. However, the actual rate of deterioration does seem to be influenced by age, those over 55 showing a high rate of up to 9 dB/decade against 3 dB/decade for those under 55. This implies that study over a much longer time is required to find a more exact form for the relationship between age and the rate of deterioration of hearing impairments.