Category: Paediatric Purpose: To study the cost-effectiveness of brain MRI compared to biochemestry and genetic testing in children with early developmental impairment (EDI). Methods and materials: Retrospective case note review of children referred to Sheffield Children's Hospital to determine the degree and cause of EDI. Haematology, biochemestry and genetics (microarray) tests and brain MRI results were reviewed and their findings divided into normal, specific diagnosis or non-specific diagnosis. Participants were divided into two groups: EDI and no additional features (EDI-) and EDI with additional features (EDI+). Results: 699 participants, 68.8% male, median age at investigation two years eight months (range three months–11 years five months). The following investigations found causes for EDI: MRI (24%), microarray (11.5%), plasma amino acids (1.2%), urine organic acids (0.9%) and thyroid function tests (0.5%). The cost per diagnosis was: £4,025 with brain MRI, £2,600 with microarray and £11,280 for the battery of blood and biochemstry tests. Conclusion: MRI has the highest diagnostic rates for EDI and is more cost effective than the battery of blood and biochemistry tests. The majority of blood and biochemistry investigations for EDI are not diagnostic, highlighting an area of potential cost saving for the NHS. We propose a new, streamlined guideline for the investigation of EDI based on our data.