The Surgeons' knot is both technically complex and difficult for junior surgeons to tie, especially when using heavy monofilament material. Knot slippage or breakage can pass unrecognized as a cause of wound dehisence, and it has been suggested that the Aberdeen knot and the Half-blood knot are easier and safer than the Surgeons' knot for tying these materials. The effect of different knots upon the breaking strain of common monofilament sutures was assessed using an Instron Tensiometer. The Surgeons' knot significantly reduced the breaking strength of all materials (P < 0.001). Using the Half-blood knot to commence closure had no significant effect upon strength. The Surgeons' knot, tied using the penultimate loop as at termination of closure, was associated with a significant risk of slippage, most noticeably with looped nylon. The Aberdeen knot, used for closure, showed no such slippage, and did not significantly weaken the suture (P < 0.05), except for polydiaxanone. In view of the ease of tying, security of knot and non-weakening properties, we recommend starting midline closure with a Half-blood knot and terminating with an Aberdeen knot.