BACKGROUND: Hormone replacement therapy (HRT) is probably unprecedented as a drug in widespread preventive use by a predominantly well population, but there is little direct data on current trends in use. AIM: To estimate trends in the use and cost of HRT in Britain. METHOD: Government prescription data and therapy cost data were analysed to provide trends in costs and point prevalence of the use of HRT since 1980. Projections were estimated to the year 2000. RESULTS: In 1987, HRT was used by an estimated 2.2% of women aged 40 to 64 years in England, and by 1.0% in Scotland. By 1994 this had risen to 21.7% in England, 20.4% in Scotland, and 21.3% in Wales. Between 1980 and 1986, costs remained steady at approximately 11 million Pounds per annum for England and 1 million Pounds for Scotland (1994 values). Between 1987 and 1994 they rose to 87 million Pounds for England, 10 million Pounds for Scotland, and 46 million Pounds for Wales (1994 values). Projections suggest that, by the year 2000, 25.4% of women aged 40 to 64 years (95% CI = 20.1-30.7%) will be taking HRT at any one time, at a cost of 150 million Pounds (95% CI = 142 m Pounds-157 m Pounds: 1994 values). This implies an 'ever' user prevalence of at least two-thirds of women in this age group. CONCLUSION: Use of HRT for long-term therapy is widespread and rising. It is estimated that prevalence has increased tenfold since 1987. Average individual costs of therapy have fallen by one third. The level of reduced risk of osteoporosis and cardiovascular disease, and the balance with cost and potential increased risk of breast cancer, are not yet established and are in need of clarification.