The combination of tamoxifen and transdermal hormone replacement therapy (HRT) may potentially reduce risks and side-effects of either agent, but an adverse interaction could attenuate their beneficial effects. We assessed the effects of their combination on cardiovascular risk factors within a prevention trial of tamoxifen. Baseline and 12-month measurements of total, low-density lipoprotein (LDL)- and high-density lipoprotein (HDL)-cholesterol, platelets and white blood cells were obtained in the following four groups: tamoxifen (n = 1117), placebo (n = 1112), tamoxifen and HRT (n = 68), placebo and HRT (n = 87). The analysis was further extended to women who were on HRT at randomization but discontinued it during the 12-month intervention period (n = 33 on tamoxifen and n = 35 on placebo) and to women who were not on HRT but started it during intervention (n = 36 in both arms of the study). Compared with small changes in the placebo group, tamoxifen was associated with changes in total, LDL- and HDL-cholesterol of approximately -9%, -19% and +0.2% in continuous HRT users compared with -9%, -14% and -0.8% in never HRT users. Similarly, there was no interaction on platelet count. In contrast, the decrease in total and LDL-cholesterol levels induced by tamoxifen was blunted by two-thirds in women who started HRT while on tamoxifen (P = 0.051 for the interaction term). We conclude that the beneficial effects of tamoxifen on cardiovascular risk factors are unchanged in current HRT users, whereas they may be attenuated in women who start transdermal HRT while on tamoxifen. Whereas a trial of tamoxifen in women already on transdermal HRT is warranted, prescription of HRT during tamoxifen may attenuate its activity.