To explore clinicians' and patients' (self)-assessment of activity overuse and underuse, and its relationship with physical capacity in patients with chronic musculoskeletal pain (CMP). Study design was cross-sectional. Participants included patients with CMP, admitted to a multidisciplinary outpatient pain rehabilitation program. The main measures used were as follows: a five-point scale to rate overuse and underuse, filled out by clinicians and patients; a five-point scale to rate physical capacity, filled out by clinicians and patients; and lifting and aerobic capacity. Cohen's [kappa] were calculated to test the agreement between assessments. Depending on the normality, a "t"-test or a Mann-Whitney "U"-test was used to test differences between the results of a capacity test and patients' and clinicians' assessments of capacity. A total of 141 patients were included: 42% were men, and 60% had back pain, 21% had neck pain, 19% had pain in a different location. Six percent of the patients rated themselves as underusers; clinicians rated 23% of the patients as underusers. Clinicians and patients fairly agreed (61%; [kappa]=0.23) in their assessments of overuse and underuse. Differences in the physical capacity of overuse and underusers, as assessed by clinicians and patients, were all nonsignificant (p greater than 0.05). The physical capacity of overusers did not differ from that of underusers (P less than 0.05). In conclusion, although clinicians and patients with CMP fairly agree on their assessment of activity overuse and underuse, the physical capacity of overusers was not different from that of underusers.