During the weekend of 20th to 22nd April 2012 seventeen clinical and basic scientists and a patient representative met in Naarden, The Netherlands to discuss Complex I deficiency. Delegates work in six ENMC member countries (Finland, France, Germany, Italy, the Netherlands and the UK) and three non-member countries (Australia, Canada and Israel). The meeting was the first ENMC workshop dedicated to Complex I deficiency, the most common subgroup of mitochondrial diseases. For outcomes of the workshop and how this will benefit patients: www.enmc.org