Methods: An open-label multicentre study was conducted in primary care centres in Spain to investigate the effect of a switch from celecoxib to rofecoxib among patients with osteoarthritis and to identify factors associated with a good response to rofecoxib treatment. Patients were eligible to participate in this study if their physicians considered that they might benefit from such a change of therapy. A total of 2228 patients (1481 women) were enrolled in the study: participants' mean age was 66.37 years (SD 9.04). Mean duration of OA was 7.44 (6.38) years. Predominant sites of OA included the knee (1132 patients, 50.8%), lumbar spine (977 patients, 43.9%) and cervical spine (739 patients, 33.2%). At baseline, most patients (77%) were being prescribed celecoxib 200 mg/day; during the study most (92.5%) received rofecoxib 25 mg/day. The mean interval between switch to rofecoxib and follow-up interview was 33 days. Principal results: Patients considered the therapeutic response to rofecoxib substantially and significantly superior to that previously obtained with celecoxib for the management of OA-related pain and OA-related health status (p < 0.001). Seventy-two percent of patients classified their response to rofecoxib therapy as 'good' or 'very good' (vs 6.6% of patients at baseline with celecoxib) and 89.3% of patients expressed satisfaction with rofecoxib (vs 28.9% at baseline with celecoxib). Improvements reported in patient self-assessments following rofecoxib therapy were complemented by similar changes in physician perceptions. The number of patients considered by their doctors to have 'good' or 'very good' OA-related health status rose from 10.1% at baseline to 80.0% on completion of rofecoxib therapy. Ancillary indices such as the proportion of patients with self-reported depression were also favourably influenced by the switch to rofecoxib from celecoxib. Determinants of response: Patient characteristics identified in multivariate analysis as predictive of a favourable response to rofecoxib comprised age, obesity, depression, diabetes and OA-related overall health status.