In an attempt to determine the optimum configuration of arteriovenous (A-V) fistulas for haemodialysis, 71 patients were prospectively randomized to undergo either a side-to-side or end-of-vein to side-of-artery A-V fistula. Nine months after operation, the patency rates on dialysis were almost identical in the two groups (79.2 per cent and 78.6 per cent respectively). However, 7 of the 32 side-to-side fistulas developed hyperaemia of the hand, three of which required revisional surgery. Hyperaemia of the hand has not been seen with end-to-side fistulas. In addition, peroperative measurements of fistula flow appeared to have prognostic value with end-to-side but not with side-to-side fistulas. It is suggested that the end-to-side configuration is the one of choice for the formation of A-V fistulas for haemodialysis.