Much is known about the serious neurological effects of gluten ingestion in coeliac disease patients, such as sporadic ataxia and peripheral neuropathy, although the causal links to gluten are still under debate. However, such disorders are observed in only a small percentage of coeliac patients. Much less is known about the transient cognitive impairments to memory, attention, executive function, and the speed of cognitive processing reported by the majority of patients with coeliac disease. These mild degradations of cognitive functions, referred to as "brain fog," are yet to be formally recognized as a medical or psychological condition. However, subtle tests of cognitive function are measurable in untreated patients with coeliac disease and improve over the first 12 months' therapy with a gluten-free diet. Such deficits also occur in patients with Crohn's disease, particularly in association with systemic inflammatory activity. Thus, cognitive impairments associated with brain fog are psychologically and neurologically real and improve with adherence to a gluten-free diet. There is not yet sufficient evidence to provide a definitive account of the mechanism by which gluten ingestion causes the impairments to cognitive function associated with brain fog, but current evidence suggests that it is more likely that the causal factor is not directly related to exposure to gluten.