The topic of Workshop W3-1 was clinical and biochemical approaches to the diagnosis of mitochondrial respiratory chain disorders. Four main questions were addressed in an attempt to make some progress towards a consensus diagnostic approach: What are the major limitations in diagnosis of respiratory chain dysfunction? What is the ideal approach to investigating children with a suspected respiratory chain disorder? Can we begin to develop consensus diagnostic criteria? Can we develop a quality assurance (QA) scheme for respiratory chain enzyme assays? The workshop demonstrated strong consensus on recognizing the limitations of current diagnostic approaches, on the ideal diagnostic approach and on the desirability of an enzyme QA scheme. There was also support for the desirability of consensus diagnostic criteria, albeit with some concerns about the practicality of gaining consensus. Two potential approaches to developing consensus criteria were described.