Fish oil may be beneficial in the treatment of psoriasis and in RA. We examined the potential benefit of Efamol Marine, a combination of evening primrose oil and fish oil in the treatment of 38 patients with PsA. Patients with PsA were entered in a double-blind placebo controlled study and received either 12 Efamol Marine capsules or 12 placebo capsules daily for 9 months. All patients received placebo capsules for a further 3 months. At month 3 of the study patients were asked to reduce their intake of NSAIDs and maintain that decrease provided there was no worsening of their joint symptoms. Clinical assessments of skin and joint disease severity and activity were performed at 0, 1, 3, 6, 9 and 12 months. All measures of skin disease activity including severity, percentage body affected and itch were unchanged by Efamol Marine. The NSAID requirement remained the same between both treatment groups. In addition, there was no change demonstrated in the activity of arthritis as measured by duration of morning stiffness. Ritchie articular index, number of active joints, ESR and CRP. However, a rise in serum TXB2 was observed in the active group during the placebo phase; in addition a fall in leukotriene B4 production occurred during the active phase period followed by a marked rise during the placebo phase suggesting some laboratory documented anti-inflammatory effect. In conclusion, this study suggests that Efamol Marine may alter prostaglandin metabolism in patients with PsA, although it did not produce a clinical improvement and did not allow reduction in NSAID requirement. A larger dose of essential fatty acid may be needed to produce a clinical benefit.