Clinical response and plasma pharmacokinetics were studied in 20 rheumatoid patients receiving three dosages of ibuprofen. There was a significant response to 1600 mg daily of ibuprofen by all three clinical measurements but increasing the daily dosage to 2400 mg produced no overall increase in response. The AUC increased with increasing daily drug dosages from 800 to 2400 mg daily and the dose normalised AUC fell by 15% over the same dosage range. The fraction of ibuprofen not bound to plasma proteins increased with increasing dosage and may contribute to the fall in the dose normalised AUC. There was a considerably inter-individual variation in the AUC. There was no significant correlation between AUC and clinical response as measured by articular index and there was a weakly significant correlation between AUC and clinical response as measured by a visual analogue pain index. Pharmacokinetic variables probably account for only a small part of the inter-individual variation in response of rheumatoid patients treated with increasing dosages of the non-steroidal, anti-inflammatory drug ibuprofen.