Conflict pervades social life. To forgive others, people must control their vengeful impulses. The efficiency by which the body uses glucose, an indicator of self-control, might have direct implications for interpersonal forgiveness. In four studies (N= 693), we tested the hypothesis that a physiological marker of deficient glucose use (i.e., type 2 diabetic symptoms) relates to less forgiveness. Higher type 2 diabetic symptoms correlated negatively with a dispositional tendency to forgive others (Study 1) and correlated positively with unforgiving motivations toward hypothetical transgressors (Study 2) and actual transgressors (Study 3). Diabetic symptoms correlated negatively with cooperative behavior in the Prisoner's Dilemma Game (Study 4). These findings provide the first evidence that forgiveness depends on how efficiently the body uses glucose. © 2010 Elsevier Ltd.