SUMMARY The most frequent cause for admission to hospital of patients on regular dialysis treatment is clotting of the arteriovenous cannula. In most instances, the cause of clotting episodes remains unknown, but several precipitating factors are recognised. Angiography of the cannulated vessels is most useful in delineating anatomical abnormalities or residual thrombus after initial de‐clotting. Local instillation of urokinase has been employed when shunt cannulae showed evidence of residual thrombus after initial de‐clotting. In 77 such episodes, the life of the treated cannula was prolonged for one to 4 weeks in 20 cases and for over 4 weeks in 24 cases.