Fish oil has fairly consistently been found to decrease rheumatoid arthritis.  

Borage Oil Helps Rheum Arthritis: A DB PC study of 34 patients for 24 weeks giving half 1.4g/day of GLA in borage oil found a 36% decrease tender joints, and similar decrease in swelling. GLA apparently decreases inflammatory fatty acids by favoring di-gamma. Leventhal, U Penn, Arch Int Med ’93;199:862; Borage oil, but not fish oil or olive oil, lowered BP and heart rate response to stress in a study. J Hum Hypertens ’89;3:111;

Evening Primrose No Better than Olive Oil for RA: DB 540/day of GLA 40 patients for 6 months. Glasgow, Br J Rheumatol 10/91 Brzeski. This is similar to borage oil.

Flax Seeds Didn't Help Rheumatoid Arthritis: A 3-month random study of flax seeds vs. sunflower seeds found no decrease in pain. AA, EPA, and DHA were all unchanged. Rheum Int ’95 14:231-4. Nordstrom DC. Ed: Flax just isn't as effective.

Fish Oil Helps Rheumatoid Arthritis in 7 DB studies: A meta-analysis of 7 DB PC studies of 395 patients found that after 3 months there was a decrease in tender joints and a decrease morning stiffness. Fortin, Harvard, J Clin Epidem 11/95;48:1379

Fish Oil Helps: Kremer, Arthritis Rheum ’95;38:1107

Cod Liver Oil in 1700s: In the 1700s, cod liver oil was routinely given to patients at the Manchester, England hospital to "oil their joints." www.lysi.is

Recommend Omega-3: LG Cleland in an editorial in the Brit J Rheum (5/97;36:513-4) says that there is evidence high omega-6 and low omega-3 increased blood clots, arrhythmias, and inflammatory disorders like RA. Says humans evolved on 2:1 ratio and not 25:1. N-6 eicosanoids promote inflammation while omega-3s retard it. Says fish oil anti-inflam and high ALA diet reduces synthesis of interleukin (IL-1 beta) and tumor necrosis factor alpha.