Background: While researchers have attempted to address the difficulties of diagnosing affective disorders in the intellectually disabled population, diagnosing bipolar disorder in an individual with severe intellectual disability (ID) remains a challenge. The aim of this study was to identify what symptoms can predict a diagnosis of mania in the intellectually disabled population. Methods: Three groups of persons with ID participated in this study: (1) individuals with a bipolar diagnosis who were currently manic; (2) individuals with an Axis I diagnosis other than bipolar disorder; and (3) individuals without an Axis I diagnosis. Two recognized measures of mania (i.e. Diagnostic Assessment for the Severely Handicapped-Revised and Parent Version of Young Mania Rating Scale) were used to evaluate symptoms of mania. A logistical regression procedure was conducted on mania items to identify which items correctly identify persons with ID who were currently manic. Results: Psychomotor agitation, decreased sleep, changes in mood and aggression were significantly related to the diagnosis of mania. Further, psychomotor agitation and disturbed sleep were significant predictors of a diagnosis of mania. Conclusions: Problems of sleep and psychomotor agitation should alert clinicians that further assessment of bipolar symptomatology is warranted. Focusing on observable behaviours based on Diagnostic and Statistical Manual of Mental Disorder-IV criteria can be useful in formulating a diagnosis of bipolar disorder in persons with ID.