In acknowledgement of the challenges posed by the diagnosis of mental disorders in people with intellectual disability (ID), the World Health Organization (WHO) has recently published a multi-axial guide to assist clinicians in applying the ICD-10 to this group of people. The WHO has invited users to review of the ICD-10 Guide for Mental Retardation. In order to review its usefulness in clinical practice, the present authors used the Guide together with the ICD-10 in the psychiatric assessment of 106 young people with ID, and emotional and behavioural problems of varying severity. Strengths and weaknesses in the utility of the Guide were noted. As a result of the clinical audit process, the present authors identified a number of inconsistencies, significant gaps and assertions unsupported by the available literature in the Guide. They also noted aspects of the ICD-10 itself which are problematic when applied to people with ID. Despite these limitations, the Guide is a valuable first attempt to establish a standardized structure for multi-axial diagnosis in this population. A number of suggestions for major changes and refinements to future editions are made, and a strategy for development of research to establish validity and reliability is proposed.