The use of postmenopausal hormone therapy after breast cancer remains controversial. Evidence shows variation by study design, and even among three randomized controlled trials there is substantial heterogeneity of results. Two Swedish trials of comparable size show relative risks of recurrence of 3.3 and 0.82 on comparing women receiving postmenopausal hormone therapy with control women. The extent of use of tamoxifen and concomitant use of progestins in combination with estrogen, although raised as one possible explanation for this heterogeneity, are not supported by evidence from trials of high-dose progestins used after breast cancer. Caution is needed when considering the use of postmenopausal hormone therapy after breast cancer.