Patients with a psychotic disorder show lower cardiorespiratory fitness and higher mortality rates compared to healthy individuals. The aim of this cross-sectional study was to explore whether in patients with first-episode psychosis a low cardiorespiratory fitness is associated with decreased physical activity and increased sedentary levels. Twenty-nine outpatients (21 men; 22.8±5.1 years) performed a maximal exercise test to assess their maximum oxygen uptake (VO2max), wore a Senswear armband for five consecutive days and were assessed with the Positive and Negative Syndrome Scale. Twenty-four patients (82.8%) scored below the normative cardiorespiratory values. The percent-predicted VO2max ranged from 47% to 109%. In a backward regression analysis, less time spent sedentary (min/day) was the only independent predictor of a higher VO2max. The model explained 28.0% of the variance in the VO2max-score. The current study indicates that future research should explore whether reducing sedentary behaviour (e.g. time spent napping or prolonged sitting during waking hours) might improve cardio-respiratory fitness levels. Interventions targeting recreational screen time (watching television, computer use, playing video games, etc.) or replacing passive to more active video games should be investigated.