Serum and synovial fluid concentrations of ketoprofen were studied after administration of 50 or 100 mg in a single oral dose to patients with rheumatoid arthritis. Results show that ketoprofen is rapidly absorbed. Serum concentrations are similar in patients and healthy subjects. During the initial phase, plasma half-life, which averaged 1.5 to 2 hours in study patients, was similar to that recorded in controls. Significant accumulation of ketoprofen in synovial fluid of patients was demonstrated.