This study compared the breaking force of acutely and chronically loaded 6-O and 5-O polypropylene sutures. It also examined the effect of graded injuries on those sutures. Studies were performed on 134 6-O and 5-O polypropylene sutures subjected to acute loads, 312 6-O and 5-O sutures subjected to chronic loads, and 377 6-O sutures subjected to quantitative "pinches" with DeBakey forceps to produce graded injuries before chronic loading. The lowest acute breaking force for 6-O polypropylene was 192 g. This occurred in 1.3% of acutely loaded sutures. The single lowest chronic breaking force for 6-O sutures was between 150 and 175 g, occurring in 5% of chronically loaded sutures. Thus, chronically loaded sutures are more likely to break than are acutely loaded ones. All of the chronically loaded sutures broke within the first 48 hr of loading, resembling the time of breakage in patients. The lowest chronic breaking force for 5-O sutures was 300 g, 71% greater than the 175 g for 6-O sutures. Graded pinches of 6-O sutures with DeBakey forceps increased the risk of breakage with chronic loads even more. All of the above data were related to the forces imposed upon a suture when it is used to close a carotid arteriotomy. Safety ratios were lower for chronically loaded 6-O sutures than for acutely loaded 6-O sutures or for chronically loaded 5-O sutures. When used to close a carotid arteriotomy, 6-O polypropylene has an acceptable safety ratio with respect to acute loads, but this is markedly reduced with chronic loading, producing a small but finite risk of breakage. The safety ratio may be reduced to dangerously low levels if the suture is handled improperly. Use of 5-O sutures would greatly increase the safety ratio of the suture line, both acutely and chronically, when polypropylene is used to close a carotid arteriotomy.