The PAS-ADD Checklist is a screening instrument specifically designed to help staff recognize mental health problems in the people with intellectual disability for whom they care, and to make informed referral decisions. The instrument consists of a life-events checklist and 29 symptom items scored on a four-point scale. Scores are combined to provide three threshold scores. The crossing of any of these thresholds indicates the need for a fuller assessment. The items are worded in everyday language, making the Checklist suitable for use by individuals who do not have a background in psychopathology. The present paper presents the results of a number of studies evaluating the reliability and validity of the Checklist. Factor analysis of Checklists completed on a community sample of 201 individuals yielded eight factors, of which seven were readily interpretable in diagnostic terms. Internal consistency of the scales was generally acceptable. Inter-rater reliability in respect to individual items gave a fairly low average Kappa of 0.42. However, agreement on case identification, the main purpose of the Checklist, was quite good, with 83% of the decision being in agreement. Validity in relation to clinical opinion was also satisfactory, case detection rising appropriately with the clinically judged severity of disorder. The PAS-ADD Checklist is published and distributed by the Hester Adrian Research Centre, Manchester, England, from where further information and order forms are available on request.