Patients undergoing haemodialysis for kidney failure are fitted with an in-dwelling catheter so that the patient’s circulation can readily be connected to the haemodialyser on each visit to hospital. Between the dialysis sessions there is a tendency for the blood within the catheter to form clots and these must be removed before dialysis can proceed. To avoid clotting a quantity of an anti-coagulant, heparin, is instilled into the line at the end of each dialysis session. This study describes a trial where heparin is compared with an alternative, alteplase, in children undergoing haemodialysis. Very few children receive haemodialysis but because those who do must attend regularly, the trial used a crossover design. While crossover designs are widely used they seldom have more than five or six periods. This article describes how a crossover trial with 30 periods was specially designed for this trial.