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The proportion of deceased donor kidneys recovered for transplant but discarded increased steadily in the United States over 2 decades, from 5.1% in 1988 to 19.2% by 2009. Over 100 000 patients are waiting for a kidney transplant, yet 3159 kidneys were discarded in 2015.We evaluated trends in donor characteristics, discard reasons, and Organ Procurement Organization–specific discard rates. Multivariable regression and propensity analysis were used to estimate the proportion of the discard rate rise in the 2000s attributable to changes in donor factors and decisions to biopsy and pump kidneys.This study found that at least 80% of the discard rate rise can be explained by the recovery of kidneys from an expanding donor pool and changes in biopsy and pumping practices. However, a residual discard rate increase could not be explained by changes in these factors. From 1987 to 2009, median donor age rose from 26 to 43 years; median Kidney Donor Risk Index increased from 1.1 in 1994 to 1.3 in 2009. Our findings suggest that the increase from 10% to 30% in the proportion of kidneys pumped during the 2000s served as a buffer, keeping the discard rate from rising even higher than it did.The majority of the kidney discard rate rise can be explained by the broadening donor pool. However, the presence of an unexplained, residual increase suggests behavioral factors (eg, increased risk aversion) and/or allocation inefficiencies may have played a role. Reducing risk aversion, improving allocation, and more often pumping less-than-ideal, yet potentially transplantable kidneys, may help reverse the trend.