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337 Summary and discussion at the ADHD outpatient clinic, 22.7% fulfilled the Rome III criteria for an FDD. Based on these results, we concluded that attention to the co-occurrence of these health problems is important and that screening for these disorders should be considered at an early stage in these patient groups. Although the pathophysiological mechanisms underlying the co- occurrence of FDDs and ADHD are incompletely understood, several factors may play a role. Children with ADHD are thought to ignore their urge to defecate or experience difficulties taking the time to complete bowel movements. 14 Children with ADHD are also less compliant to incontinence treatment, which may play a role. 15 Moreover, we hypothesized that in patients with ADHD, an altered communication between the central nervous system (CNS) and the enteric nervous system could potentially result in disturbed gastrointestinal motility. 16 The importance of brain-gut interactions in the regulation of digestive processes, including the regulation of bowel movements, is increasingly recognized and confirmed in studies utilizing functional magnetic resonance imaging. 17–21 Compared to healthy controls, adults with FC have been reported to display significantly different baseline activity in a number of major brain regions implicated in emotional process modulation, somatic and sensory processing, and motor control. 21 Moreover, in children with FC, patterns of cerebral activation and deactivation during rectal distension have been shown to differ from healthy controls whereas brain processing of rectal distension in FNRFI patients resembles that of healthy controls. 20 Future studies utilizing fMRI in FDD patients with and without ADHD and in ADHD patients with and without FDDs could potentially shed more light on this issue and are therefore of interest. In part II of this thesis, studies concerning the evaluation of children with FC are discussed. In chapter 5 we aimed to evaluate the agreement between two commonly used methods for assessment of stool consistency among parents of infants and toddlers: verbal report and the Bristol Stool Form Scale (BSFS). Parents of 1,095 children under 4 years of age seen for a well-child visit completed a questionnaire about their child's bowel habits during the previous month. We compared parental verbal report and the BSFS in the assessment of stool consistency and concluded that only fair agreement existed between these two methods of stool consistency assessment. Although the BSFS is commonly used in children of all ages, its reliability as an assessment tool in young children who are not toilet trained has been debated. 22,23 Our study shows that verbal report of stool consistency and assessment according to the BSFS produce different outcomes, which may be of relevance, especially when stool consistency is used as a diagnostic criterion for the diagnosis of FC or as an outcome measure in clinical trials. 24–26 However, our study does not conclude which method of stool consistency is more appropriate, our data do not allow us to determine a gold standard method of stool consistency assessment. Future endeavors to evaluate why verbal report differs from the BSFS and which method is most suitable are needed. The challenges regarding stool consistency assessment are not limited to young children; in