Twenty patients with primary osteoarthritis (OA) of the knee completed self-administered questionnaires probing pain severity on 10 occasions every day for 7 consecutive days. Rhythmometric analyses detected a significant circadian rhythm in pain in the majority of patients. Our data suggest that perceived pain severity varies substantially with time of day. These observations have important implications for the administration of pain questionnaires as outcome measures for clinical trials of nonsteroidal antiinflammatory drugs in OA of the knee.