Objectives: To assess the change over time in the physical activity level after a spinal cord injury (SCI), to explore its determinants, and to compare the physical activity level I year after discharge from the rehabilitation center with the level in able-bodied persons. Design: Prospective cohort study. Measurements were obtained at the start of active rehabilitation, 3 months later, at discharge, 2 months after discharge, and 1 year after discharge. Setting: Rehabilitation center in The Netherlands and the participant's home. Participants: Persons (n=40) with SCI. Interventions: Not applicable. Main Outcome Measures: The physical activity level, as indicated by the duration of dynamic activities (ie, wheelchair driving, walking, cycling, noncyclic movement) per day, and the intensity of everyday activity; both were measured with an accelerometry-based activity monitor during 2 consecutive weekdays. Results: Random coefficient analyses showed that the duration of dynamic activities and the intensity of everyday activity increased during inpatient rehabilitation at rates of 41% and 19%, respectively (P <.01). Shortly after discharge, there was a strong decline (33%; P <.001) in the duration of dynamic activities. One year after discharge, this decline was restored to the discharge level but was low in comparison with levels in able-bodied persons. The level of lesion and completeness of lesion were determinants of the change in the physical activity level after discharge. Conclusions: The physical activity level increased during inpatient rehabilitation, but this increase did not continue after discharge, and the level I year after discharge was distinctly lower than the level in able-bodied persons. Subpopulations had a different change over time in the physical activity level after discharge.