This study evaluated whether supplementation with olive oil could improve clinical and laboratory parameters of disease activity in patients who had rheumatoid arthritis and were using fish oil supplements. Forty-three patients were investigated in a parallel randomised design. Patients were assigned to one of three groups. In addition to their usual medication, the first group (G1) received placebo (soy oil), the second group (G2) received fish oil omega-3 fatty acids (3 g/day) and the third group (G3) received fish oil omega-3 fatty acids (3 g/day) and 9.6 ml of olive oil. Disease activity was measured by clinical and laboratory indicators at the beginning of the study and after 12 and 24 weeks. Patients’ satisfaction in activities of daily living was also measured. There was a statistically significant improvement in G2 and G3 in relation to G1 with respect to joint pain intensity, right- and left-hand grip strength after 12 and 24 weeks, duration of morning stiffness, onset of fatigue, Ritchie’s articular index for pain joints after 24 weeks, ability to bend down to pick up clothing from the floor, and getting in and out of a car after 24 weeks. G3, but not G2, in relation to G1 showed additional improvements with respect to duration of morning stiffness after 12 weeks, patient global assessment after 12 and 24 weeks, ability to turn taps on and off after 24 weeks and rheumatoid factor after 24 weeks. In addition, G3 showed a significant improvement in patient global assessment in relation to G2 after 12 weeks.