Long-term endocrine therapy for breast cancer may have clinical implications as drugs that potentially alter the lipid profile may increase the risk of developing cardiovascular disease. In this study, a companion subprotocol to the ATENA (Adjuvant post-Tamoxifen Exemestane versus Nothing Applied) trial, we compared the effect of the steroidal aromatase inactivator exemestane on the lipid profile of post-menopausal women with operable breast cancer in the adjuvant setting to that of observation alone following deprivation of 5-7 years primary treatment with tamoxifen. In this open-label, randomized, parallel group study, 340 post-menopausal patients with operable breast cancer who had been treated with tamoxifen for 5-7 years were randomized to either 5 additional years of exemestane (25 mg/day; n=172) or observation alone (n=168). Assessments of total cholesterol, high-density lipoprotein (HDL), low-density lipoprotein (LDL) and total serum triglycerides (TRG) were performed at baseline, and at 6 and 12 months. Total TRG levels were significantly reduced compared with baseline for the exemestane and the observational arm. Both total cholesterol and LDL levels were significantly increased above that of baseline values by 6 months, maintained through to 12 months, with no significant difference between the two treatment arms. There was no significant alteration observed for HDL over time or between the two arms. We conclude that sequential adjuvant treatment with exemestane in post-menopausal operable breast cancer patients following cessation of 5-7 years of tamoxifen does not appear to significantly alter the lipidemic profile for at least 12 months compared with an observational arm.