Objective. To compare the total amount of physical activity (TPA) and time spent in various activity intensities of patients with ankylosing spondylitis (AS) and population controls, and to explore factors related to physical activity (PA). Methods. Subjects were asked to wear a triaxial accelerometer for 7 days and to complete a series of questionnaires. Multivariable regressions were used to assess generic determinants of TPA in patients and controls, and in patients to explore demographic and disease-specific determinants of various PA intensities. Results. One hundred and thirty-five patients [51 +/- 13 yrs, 60% men, body mass index (BMI) 26.0 +/- 4.3 kg/m(2)] and 99 controls (45 +/- 12 yrs, 67% men, BMI 25.1 +/- 4.3 kg/m(2)) were included. Patients did not differ from controls regarding TPA (589 vs 608 vector count/min, p = 0.98), minutes/day spent in sedentary (524 vs 541, p = 0.17), and light PA (290 vs 290 p = 0.95), but spent fewer minutes/day in moderate to vigorous PA (MVPA; 23 vs 30 min/day, p = 0.006). Perceived functional ability (physical component summary of the Medical Outcomes Study Short Form-36) and BMI were associated with TPA independent of having AS (p interaction = 0.21 and 0.94, respectively). Additional analyses in patients showed that time spent in MVPA was negatively influenced by BMI, physical function (Bath AS Functional Index), and disease duration. In patients >= 52 years old, a higher Bath AS Disease Activity Index was associated with less time spent in sedentary and more time spent in light activities. Conclusion. Compared with controls, patients with AS had similar TPA, but may avoid engagement in higher intensities of PA. Lower levels of functional ability and higher BMI were associated with lower TPA in both patients and controls.