The therapeutic principles covering sex hormone replacement therapy after menopause have undergone profound modification in recent years. Initial optimism regarding benefits, proven and unproven, was followed by deep pessimism because of potential serious adverse effects. Some degree of equilibrium has resulted from the application of risk-benefit and cost-effectiveness formulae to such hormone replacement regimens. Cost-effectiveness analysis in particular has highlighted the fact that different hormones or hormone combinations can markedly affect the therapeutic outcome. The purpose of the present paper is to examine the place of oestriol therapy after menopause based on such risk-benefit analyses. Oestriol, it would appear, has the potential for reduced risk but similar benefit to alternative oestrogen or oestrogen-progestogen combinations. The potential risks and benefits of long-term oestrogen therapy are therefore surveyed from the general standpoint of all oestrogens and the specific role of oestriol alone.