Patients with chronic low back pain (CLBP) often report a disabling decrease in their activity level due to pain. The nature of the association between disability, activity, and pain over time is however, unclear. An intriguing issue here is whether a high level of pain-related disability is associated with a low activity level or are changes in the level of activity over time pain provoking and thus more disabling? The objectives of this study were to investigate associations between disability, pain intensity, pain-related fear, and characteristics of physical activity in patients with CLBP. A total of 42 patients with CLBP were recruited from the Pain Clinic of the Maastricht University Hospital. Each pain patient carried an electronic diary for one week, in which questions about current pain intensity, and the level of physical activity were completed at 8 moments a day. Disability was scored by the Quebec Back Pain Disability Scale (QBPDS), Fear of movement by the Tampa Scale for Kinesiophobia (TSK). To explain the level of disability regression analyses were performed with disability as dependent variable and pain intensity, pain-related fear, and consecutively the level of physical activity in daily life and fluctuations in physical activity as independent variables. Results, based on 34 patients, showed that activity fluctuations (beta = 0.373, p <0.05) rather than the mean activity level over time (beta = -0.052, ns) contributed significantly in explaining disability. The results are discussed in the light of current theories, previous research, and clinical implications.