This text contains papers delivered at the 33rd Annual Symposium of the Society for the Study of Inborn Errors of Metabolism, held in Toledo, Spain in 1995. Several contributions provide the biochemical background to energy metabolism. These include papers on metabolic fuel utilisation in different tissues (Medina) and mitochondrial function and biogenesis (Clayton), and in addition a general description of the neuropathogenesis of lactic acidosis (Brown). Mitochondrial defects which cause a large number of disorders of lactic acid metabolism are covered in a series of papers which discuss disorders of the electron transport chain (Robinson), molecular genetic studies (Di Mauro), neurological (Zevaini) and non-neurological presentation (Munnich). Clinical, laboratory and radiological investigation of patients with lactic acidosis are well described in papers on morphology (Romero), sampling and interpretation of metabolic intermediates (Saudubray), imaging (Van der Knaap) and spectroscopy (Gadian). Other disorders of lactate metabolism are reviewed in papers on gluconeogenesis (Van den Berghe), pyruvate carboxylase and pyruvate dehydrogenase (Robinson). Advances in treatment (Leonard) and prenatal diagnosis and genetic counselling (Ruitenbeek) are also covered. Associated with the latter are contributions on genetic screening (Scriver), gene therapy (Eisensmith) and ethical problems in genetic screening (Estevil). Also in this volume is the Society's Komrower lecture on Inborn Errors of Cholesterol Biosynthesis (Salen).