We investigated treatment effects by oestrogen receptor (ER) status among women with metastatic breast cancer (MBC) receiving capecitabine (C) plus docetaxel (D) or D alone in a randomised phase III trial. Data were retrospectively analysed from patients whose disease had recurred following (neo)adjuvant anthracyclines. ER status was identified in 356/506 patients. In patients with ER-positive tumours, median overall survival from enrolment was 17.7 months with CD versus 12.5 months with D (hazard ratio [HR] 0.65, 95% confidence interval [CI]: 0.47–0.89; P = 0.007) and median time to progression (TTP) was 6.8 and 5.4 months, respectively (HR 0.62, 95% CI: 0.46–0.84; P = 0.002). For patients with ER-negative tumours, significantly longer TTP was seen with CD (5.2 versus 3.5 months; HR 0.73, 95% CI: 0.53–0.98; P = 0.038). Whether there is an additional C to D treatment benefit in ER-positive versus ER-negative MBC requires further evaluation.