A total of 80 patients with chronic, stable psoriasis, 34 of whom also had psoriatic arthritis, were treated with 1122 mg/day eicosapentaenoic acid ethyl ester and 756 mg/day docosahexaenoic acid ethyl ester. Before the study and after 4 and 8 weeks of treatment a Psoriatic Association scoring index (PASI) score was assessed. Before treatment the mean PASI score was 3.56, after 4 weeks 1.98 and after 8 weeks 1.24; the decrease in the score was highly significant (P less than 0.001). The degree of pruritus decreased most rapidly, followed by scaling and induration of the plaques, and erythema was most persistent. At the end of the trial, seven patients were completely healed and in 13 other patients more than 75% healing was observed but in 14 patients the result was poor. The majority of patients with psoriatic arthritis reported a subjective improvement in joint pain during the study. It is concluded that polyunsaturated ethyl ester lipids may be useful for the treatment of psoriasis and psoriatic arthritis and may provide an important adjuvant to standard therapy of both conditions.