The study assessed the validity of an intellectual disability screening tool, the Learning Disability Screening Questionnaire (LDSQ), in three forensic settings: a community intellectual disability forensic service; a forensic in-patient secure unit and a prison, using data for 94 individuals. A significant positive relationship was found between full scale IQ and LDSQ score, indicating convergent validity. Discriminative validity was indicated by, firstly, a significant difference in the LDSQ scores between those with and without an intellectual disability, with those with a diagnosis of intellectual disability, scoring significantly lower. Secondly, a ROC analysis indicated that the sensitivity and specificity of the LDSQ were both above 80%. The screening tool was found to have lower sensitivity in the forensic populations than was obtained in the original community standardisation sample, but had slightly higher specificity. Limitations and implications of the study are discussed.