A double-blind randomized parallel-group trial was undertaken to evaluate the influence of the dosing time of sustained-release ketoprofen (SRK) on its acceptability and efficacy. The SRK was prescribed for 2 weeks (200 mg once a day) to 117 outpatients with osteoarthritis of the knee and/or hip. One group received SRK in the morning (at 8 a.m.) and the other group in the evening (at 8 p.m.). The principal aim of the trial concerned the acceptability, whereas efficacy was its secondary aim. The principal trial criterion was defined as the number of spontaneous recordings of adverse effects. Results showed clearly that the acceptability of SRK in the SRK morning group was worse than that in the evening group (39% of patients with one or more adverse effects in the SRK morning group versus 19% in the evening group; p = 0.019). It is important to stress the difference concerning the number of adverse effects (48 for SRK morning group versus 23 for SRK evening group; p = 0.0234). The analgesic efficacy seemed to be similar, but one criterion was statistically significant: The duration of analgesic efficacy was more important for the SRK evening group than for the morning group (9.37 and 5.47 h, respectively; p = 0.001). To increase its acceptability, evening administration of SRK seems to be preferred over morning administration in osteoarthritis. However, other trials of the same type, assessing other antiinflammatory agents, are necessary before a general extrapolation of such results can be undertaken.