Females with PTEN Hamartoma Tumor Syndrome (PHTS) have breast cancer risks up to 76%. This study assessed associations between breast cancer and lifestyle in European female adult PHTS patients. Data were collected via patient questionnaires (July 2020–March 2023) and genetic diagnoses from medical files. Associations between lifestyle and breast cancer were calculated using logistic regression corrected for age. Index patients with breast cancer before PHTS diagnosis (breast cancer index) were excluded for ascertainment bias correction. In total, 125 patients were included who completed the questionnaire at a mean age of 44 years (SD = 13). This included 21 breast cancer indexes (17%) and 39 females who developed breast cancer at 43 years (SD = 9). Breast cancer patients performed about 1.1 times less often 0–1 times/week physical activity than ≥2 times (ORtotal-adj = 0.9 (95%CI 0.3–2.6); consumed daily about 1.2–1.8 times more often ≥1 than 0–1 glasses of alcohol (ORtotal-adj = 1.2 (95%CI 0.4–4.0); ORnon-breastcancer-index-adj = 1.8 (95%CI 0.4–6.9); were about 1.04–1.3 times more often smokers than non-smokers (ORtotal-adj = 1.04 (95%CI 0.4–2.8); ORnon-breastcancer-index-adj = 1.3 (95%CI 0.4–4.2)); and overweight or obesity (72%) was about 1.02–1.3 times less common (ORtotal-adj = 0.98 (95%CI 0.4–2.6); ORnon-breastcancer-index-adj = 0.8 (95%CI 0.3–2.7)). Similar associations between lifestyle and breast cancer are suggested for PHTS and the general population. Despite not being statistically significant, results are clinically relevant and suggest that awareness of the effects of lifestyle on patients’ breast cancer risk is important.