Recent research has shown that people with chronic illnesses often experience cognitive deficits, such deficits may be specific to a particular type of illness, reflecting the disease process itself, or they may be deficits that are common across a number of chronic illnesses. Our study investigated whether people with an organic disease (Inflammatory Bowel Disease) show cognitive dysfunction relative to the control group and people with a functional illness (Irritable Bowel Syndrome), and if so, to elucidate the mechanisms by which such dysfunction occurs. A quasi-experimental design using three groups of participants provided scores on IQ, memory, and cognitive flexibility. Differences in absolute scores were slight. However, a noticeable interaction effect was found between group and IQ: The illness groups showed a deficit in verbal IQ relative to both their own performance IQ and to that of the control group's verbal IQ. This verbal deficit cannot be explained by depression, cognitive load, or medication.