Sham and real trials of peritoneal dialysis were carried out in a double-blind crossover study design to test the efficacy of peritoneal dialysis as a treatment for psoriasis. The criteria for patients included having severe plaque-type psoriasis unresponsive to all conventional therapy including methotrexate. Patients were randomly assigned to 4 weeks of sham or real dialysis, 8 weeks of observation, 4 weeks of alternative real or sham dialysis and 8 weeks of observation. Topical therapy was continued during the trial. Real dialysis was done for 48 hours weekly with 1.5-hour cycle times and 2-litre exchanges by machine. Sham peritoneal dialysis was done recycling the same 2 litres of 1.5% continuous ambulatory peritoneal dialysis fluid with 1.5-hour cycle times for 48 hours weekly. After real dialysis, two patients completely cleared, two patients had greater than 75% clearing, and one patient had no substantial response. None of the five patients had a response to the sham dialysis procedure (p less than 0.01).