Women at increased breast cancer (BC) risk are eligible for chemoprevention. Healthy lifestyles are potentially important for these women to improve efficacy and minimise side effects of chemoprevention and reduce the risk of BC and other lifestyle-related conditions. We investigated whether women taking chemoprevention adhere to healthy lifestyle recommendations, how their lifestyle risk factors and health measures compare to women in the general population, and whether these change whilst taking chemoprevention. Lifestyle risk factors and health measures in 136 premenopausal women taking tamoxifen for prevention of BC (Tam-Prev study) were compared to both national recommendations and an age-matched female population from the Health Survey for England 2012. The Tam-Prev population had high rates of overweight and obesity (59.2%) and low adherence to physical activity recommendations (30.6%) which were comparable to the general population (55.2 and 35.1%, respectively). Fewer Tam-Prev participants were current smokers (10.5 vs. 18.2%, P = 0.032), but more exceeded alcohol recommendations (45.0 vs. 18.7%, P < 0.001). Tam-Prev participants had suboptimal diets; proportions not meeting fibre, saturated fat and non-milk extrinsic sugar recommendations were 87.8, 64.9 and 21.4% respectively. Many Tam-Prev participants had markers of cardiovascular disease risk and the metabolic syndrome. Health behaviours did not change during the first year on tamoxifen. Women taking chemoprevention had a high prevalence of unhealthy lifestyle behaviours and health measures, similar to an age-matched English cohort. Improving these measures in women at increased BC risk could significantly decrease rates of BC and other noncommunicable diseases.