Four experiments demonstrate that self-knowledge provides a mixed blessing behavioral prediction, depending on how accuracy is measured. Compared with predictions of others, self-knowledge tends to decrease overall accuracy by increasing bias (the mean difference between predicted behavior and reality) but tends to increase overall accuracy by also enhancing discrimination (the correlation between predicted behavior and reality). Overall, participants' self-predictions overestimated the likelihood that they would engage in desirable behaviors (bias), whereas peer predictions were relatively unbiased. However, self-predictions also were more strongly correlated with individual differences in actual behavior (discrimination) than were peer predictions. Discussion addresses the costs and benefits of self-knowledge in behavioral prediction and the broader implications of measuring judgmental accuracy of judgment in terms of bias versus discrimination.