The knot performance of four different nonabsorbable sutures-silk, polyester, polyamide, and polypropylene-is investigated by using square knots with two and three throws. All sutures are USP 2/0 in size. An instrument that can be mounted on the Instron tensile tester measures the knot performance of the sutures. The results are given as stress-elongation graphic, knot slippage, and knot break values. The effects of the number of throws and the structure of the sutures on knot performance are studied. For the knot performance tests, the effect of the physical structure (braided or monofilament) is clearly seen. For monofilament polyamide and polypropylene sutures, the knot with two throws unties before it breaks. For silk and polyester braided sutures, which have reasonably good two-throw square knot strength, the added throw does not remarkably increase strength and at the same time places more suture material in the tissue, which increases the risk of infection. Three-throw knot results reveal that the suture material plays an important role in knot strength.