According to the Memory for Past Test (MPT) heuristic, judgments of learning (JOLs) may be based, in part, on memory for the correctness of answers on a previous test. The authors explored MPT as the source of the underconfidence with practice effect (UWP; A. Koriat, L. Sheffer, & H. Ma'ayan, 2002), whereby Trial 1 overconfidence switches to underconfidence by Trial 2. Immediate and delayed JOLs were contrasted because only immediate JOLs demonstrate UWP. Consistent with MPT for immediate JOLs, Trial 1 test performance better predicted Trial 2 JOLs than did Trial 2 test performance. Delayed JOLs showed the reverse. Furthermore, items forgotten on Trial 1 but remembered on Trial 2 contributed disproportionately to UWP, but only with immediate JOLs.