Physical activity (PA) helps prevention and aftercare of sporadic breast cancer (BC), cardiopulmonary fitness (CPF) being an age-independent predictor of tumor-specific mortality. Therefore, we wanted to identify predictors of CPF (represented by peak oxygen uptake: VO ) in BRCA1/2 mutation carriers whose risk of developing BC is high. We used cross-sectional data from 68 BRCA1/2 germline mutation carrying women participating in the randomized, prospective, controlled clinical study LIBRE-1. Assessments included cardiopulmonary exercise testing, medical and lifestyle history plus socioeconomic status. Additionally, the participants completed a psychological questionnaire regarding their attitude, subjective norms, perceived behavior control and intention towards PA. A multivariate logistic regression model was used to identify predictors for participants reaching their age- and sex-adjusted VO reference values. 22 participants (median age: 40 years, interquartile range (IQR) 33-46) were cancer-unaffected and 46 cancer-affected (median age: 44 years, IQR 35-50). The strongest predictor for reaching the reference VO value was attitude towards PA (Odds Ratio 3.0; 95% Confidence Interval 1.3-8.4; p = 0.021). None of the other predictors showed a significant association. A positive attitude towards PA seems to be associated with VO , which should be considered in developing therapeutic and preventive strategies.Trial registrations: NCT02087592; DRKS00005736.