Background: There is a history of over-prescription of antipsychotics to individuals with intellectual disability (ID), while antidepressants may be under-prescribed. However, appropriate treatment is best supported when the diagnosis of psychosis or depression is valid and carries good predictive validity. The present authors report a study examining one aspect of validity, namely whether skilled clinicians can agree on whether an individual with an ID is psychotic or depressed. Materials and Methods: Pairs of clinicians assessed 52 individuals. Agreement was assessed using Cohen's kappa statistic and agreement proportion. Results: Overall agreement was high for both psychosis and depression. Whether the individual had mild ID or moderate/severe ID did not have a significant impact on agreement. Conclusions: Experienced clinicians achieved a high level of agreement as to whether a person with ID was psychotic or depressed similar to that found for those without ID. The findings provide some support for treatment interventions based on diagnosis.