【Abstract】 Objecitve To investigate the causes and prevention of aneurysm-like dilatation of internal arteriovenous fistula in hemodialysis patients. Methods A total of 350 patients treated with hemodialysis for 1~32 years in this hospital were recruited and divided into two groups. Patients in group A had aneurysm-like dilatation of internal arteriovenous fistula, and those in group B had no dilatation. Local or multistep puncture to the fistula were applied during hemodialysis. After dialysis, adhesive tape round the arm combined with pressure by fingers or elastic bandage were used on the puncture site. Results Aneurysm-like dilatation of arteriovenous fistula (group A) was found in 42 cases (12%), of which 29 cases (69%) had hypertension, similar to the hypertension incidence (61%, P＞0.05) in group B. In the 42 patients with the dilatation, 24 (57.1%) cases were treated with hemodialysis for more than 10 years; fistula was in forearm in 35 cases, in upper arm in 6 cases, and PTFE graft in one case. Aneurysm-like dilatation presented at puncture site in 29 cases, at anastomotic site in 8 cases, along the venous portion in 3 cases, and as pseudoaneurysm in 2 cases. Local puncture was applied in 69% cases in group A, but in 21% in group B (P<0.01). About 2/3 patients with fistula in upper arm were complicated with the aneurysm-like dilatation. The way of anastomotic operation, pressure on puncture site after dialysis, and plasma calcium, phosphorus and parathyroid hormone (PTH) had no differences between two groups. Conclusion The causes of aneurysm formation of arterialvenous fistula may relate to repeated punctures at local site, the site of fistula, hypertension and vascular pathological changes. Prevention of aneurysm-like dilatation is essential to the persistent protection of the fistula