In a comparative double-blind trial involving 263 postmenopausal women with advanced breast cancer treated with tamoxifen, the mean objective tumour response rate and duration was 32% and 15 months respectively. No significant difference was found in clinical response and adverse effects between those randomised to 10 mg and those to 20 mg twice daily. Although the mean serum concentration of tamoxifen in the 20 mg bd group was significantly higher no correlation between serum level and clinical benefit was demonstrated.