Response monitoring (post-error slowing) and inhibitory control (commission errors) were examined in children with phenylketonuria (PKU) and controls (6-18 years) using Go/No-Go tasks with higher (PKU n = 37; control n = 55) versus lower (PKU n = 24; control n = 25) non-target expectancy. On both tasks children with PKU exhibited impaired monitoring and inhibitory control, but the post-error slowing pattern was different. With higher expectancy children with PKU slowed more (less efficient monitoring) and with lower expectancy slowed less (less monitoring) than controls. No effects of age or phenylalanine level were noted. These results indicate that expectancy differentially effects monitoring and inhibitory control in PKU.