ABSTRACTBackgroundThis study compared the physical activity level of men with metastatic prostate cancer at baseline of the multicentre INTERVAL‐GAP4 trial to the American Cancer Society guidelines and examined associations with physical fitness.MethodsA total of 140 men on androgen deprivation therapy (ADT) were included in this cross‐sectional analysis of baseline data from the INTERVAL‐GAP4 trial. Exclusion criteria included a maximum of 1 h of vigorous aerobic exercise or one structured resistance exercise session per week but no restrictions on habitual physical activity. Moderate‐to‐vigorous physical activity (MVPA) was assessed using a modified Godin–Shephard Leisure‐Time Physical Activity Questionnaire. Physical fitness measurements included peak oxygen consumption (VO2peak), maximal power output (Wmax), 400 m walk time, one‐repetition maximum (1RM) of leg extension, leg press, chest press and seated row, and handgrip strength. Quantile regression was used to analyse associations of MVPA with physical fitness outcomes at the 25th, 50th and 75th percentiles of the physical fitness distributions.ResultsTotal self‐reported MVPA was 60 (IQR: 0, 180) min per week, with 29% meeting the aerobic physical activity guidelines. There was a statistically significant association of higher MVPA with higher relative VO2peak at the 25th (β = 0.53, p = 0.020) and 75th percentiles (β = 0.66, p = 0.001), relative Wmax at the 25th (β = 0.05, p = 0.003), 50th (β = 0.05, p = 0.009) and 75th percentiles (β = 0.07, p = 0.004) and reduced 400 m walk time at the 75th percentile (β = −4.26, p = 0.023), with β corresponding to the change in the dependent variable for each one‐hour increase in weekly MVPA.ConclusionFew men recruited to the INTERVAL‐GAP4 trial were meeting aerobic physical activity guidelines at baseline. Higher MVPA was associated with better aerobic capacity and walking performance but not maximal strength in men with metastatic prostate cancer on ADT.Trial RegistrationClinicalTrials.gov: NCT02730338; German Clinical Trials Register: DRKS00010310