This study examines the rate of psychiatric diagnosis as per ICD-10 and Reiss Screen for Maladaptive Behaviours (RSMB), and distribution of psychiatric diagnosis with regard to the severity of intellectual disability (ID). It also explores the degree of agreement between Reiss screen and clinical diagnosis (ICD-10) in relation to dual diagnosis. In this study Intelligence (IQ) and Social Quotient (SQ) were estimated on Indian adaptation of Binet's scale and Vineland Social Maturity Scales, respectively, in 60 consecutive persons with ID. Both ICD-10 and RSMB were used independently to determine the presence of psychiatric diagnosis. Clinically, according to ICD-10, about 60% of the sample was found to have dual diagnosis as compared to 48% on RSMB. Agreement between ICD-10 and RSMB about psychiatric comorbidity was 82%. Commonest psychiatric diagnosis was unspecified psychosis followed by bipolar affective disorders. Persons with ID do suffer from various psychiatric disorders. RSMB is a useful tool for differentiating between psychiatric problems and maladaptive behaviours. Hence RSMB can be used in the Indian context.