Intolerance of uncertainty (IU) has often been studied in the context of internalizing disorders, but no studies to our knowledge have explored the relation between IU and externalizing disorders. Given the proposed link between IU and emotion regulation, the current study sought to examine levels of IU in an externalizing clinical population with known emotion regulation difficulties—ADHD. IU levels in this population were compared to a clinical population known to experience elevated levels of IU – anxiety disorders. Ninety-three children (36 anxiety disorder, 28 ADHD, 29 unaffected children) ages 7-13 participated in present study. Children completed the Intolerance of Uncertainty Scale – Short Version (IUS). Responses on the IUS were converted to total IU, prospective IU and inhibitory IU. Analyses of Covariance (ANCOVAs) were conducted controlling for age, sex, and ADHD medications. Significant differences were observed between child diagnostic status and total IU (F(2, 87) = 4.43, p = .02), prospective IU (F(2, 87) = 5.57, p = .005), and inhibitory IU (F(2, 87) = 3.86, p = .025). Pairwise comparisons revealed that children with anxiety disorders demonstrated greater total IU relative to unaffected children (F(1,87) = 8.85, p = .004), and greater prospective IU relative to unaffected children (F(1,87) = 9.46, p = .003) and ADHD children (F(1,87) = 6.04, p = .016). Children with ADHD demonstrated greater inhibitory IU relative to unaffected children (F(1,87) = 6.08, p = .016). No significant differences were observed between children with anxiety disorders and children with ADHD for total IU (F(1,87) = 1.49, p = .23) and inhibitory IU (F(1,87) = .31, p = .58). These findings contribute to a growing literature on the link between IU and psychopathology. IU appears to be a transdiagnostic construct present among children with internalizing and externalizing disorders, and results further substantiate literature suggesting IU may be more broadly associated with emotion regulation deficits.