The advent of functional neuroimaging raises the intriguing possibility that investigators might be able to determine (one day) whether an individual is lying or telling the truth, according to the activity of their brain. Ultimately, such techniques might be applied in the forensic sphere. However, the empirical data supporting this conjecture derive from a body of work that is still early on in its development. Hence, when invited to play ‘Devil's advocate’, the author is prompted to critique a pivotal weakness within the current literature. The latter comprises 16 peer-reviewed functional magnetic resonance imaging studies purporting to describe the neural correlates of lying. Most have demonstrated greater activation of prefrontal regions while participants lie relative to when they tell the truth. Most have failed to detect areas where truthfulness elicits specific activation (consistent with the view that truthfulness constitutes a ‘baseline’ in human cognition and communication; while lying requires something more). However, there is a great deal of variation between the findings described and, crucially, there is an absence of replication by investigators of their own findings. Hence, basic issues of reliability need to be addressed before functional neuroimaging is applied to cases that matter in the ‘real world’.