Endocrine therapy plays a major part in the systemic treatment of common cancers such as breast and prostatic carcinomas and some gynaecological malignancies. Although more than two million women have been treated with the anti-oestrogen tamoxifen, its disposition is still not completely understood. The absorption fractions for tamoxifen and the progestin drugs are uncertain. Many studies evaluating the plasma pharmacokinetics of tamoxifen and progestin drugs used blood sampling times that may have been too short to allow the determination of the terminal half-life with certainty. For most drugs, information on tissue concentration in general and tumour concentration in particular is lacking. Clinical results obtained with combined treatment may have been confounded by drug interactions, as exemplified by use of the enzyme inducer aminoglutethimide. Aims for further studies will be to compare tissue distribution and tumour distribution in relation to treatment response and to evaluate drug interactions. Increasing use of tamoxifen in premenopausal women highlights the need to study long term effects with special regard to possible teratogenicity.