BACKGROUND: Despite beneficial effects of conjugated linoleic acids (CLAs) in animal studies, there is little information on their effects on human inflammatory and autoimmune diseases. AIM: To investigate the effects of CLAs as an adjuvant therapy on the clinical manifestations of rheumatoid arthritis (RA) in adults with an active disease. METHODS: In a randomized, double-blind placebo-controlled trial, 87 patients with active RA were divided into four groups receiving one of the following daily supplements for 3 months: group C: CLAs 2.5 g equivalent to 2 g mixture of cis 9-trans 11 and trans 10-cis12 CLAs at a rate of 50/50; group E: vitamin E: 400 mg; group CE: CLAs and vitamin E at above doses; group P: placebo. Serum alpha-tocopherol was determined by high-performance liquid chromatography. Clinical data was determined by physician examination and filling the questionnaire by interview. Complete blood count (CBC), erythrocyte sedimentation rate (ESR), C-Reactive protein (CRP) and rheumatoid factor (RF) were measured in each patient. DAS28 (diseases activity score) was also determined. RESULTS: A 3-month supplementation resulted significant reduction in DAS28, pain and morning stiffness in the groups C and CE compared with group P (P<0.05). Compared with the baseline, ESR levels decreased significantly in the groups C (P0.1). The reduction of white blood cell count was significant in group CE compared with other groups (P<0.05). Decrease in platelet count was non-significant in groups CE, C, and E. Changes in RF, body mass index, red blood cell count and hemoglobin were not significant in four groups, while RF decreased non-significantly in groups CE and E. In comparison with the baseline, alpha-tocopherol increased significantly in groups C (P