In this study 383 mammograms of women on hormone replacement therapy (HRT) for more than one year and 216 mammograms of women performed before HRT was started were examined to identify any mammographic features which are associated with a higher incidence of developing breast cancer according to Wolfe's criteria. There was no significant difference in the proportion of high and low risk parenchymal patterns as the duration of HRT increased. A significantly higher proportion of low risk patterns with ageing was observed in 216 women who had never been on HRT when compared to 194 mammograms of women who had been on HRT for more than 5 years. This difference becomes more obvious in women on HRT with less than three children compared to a similar group not taking HRT. Women who had not taken HRT appeared to undergo normal involutional changes which were reflected by an increase in the proportion of low risk patterns with increasing age. We may conclude from these findings that HRT appears to inhibit involutional processes within the breast and therefore this group have higher risk parenchymal patterns for a longer period of time and subsequently may have a higher risk of developing breast cancer.