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Analytical biochemical and molecular genetic studies, based on biopsies of muscle, skin and tissue culture have become standard methods for a more precise determination of mitochondrial defects. It is hoped that from these studies some precise classification of mitochondrial diseases will arise in the future. At present the information is incomplete due to a series of problems faced by mitochondrial researchers, not accustomed to deal with the precise requirements of human biopsy analytical methodology and with the lack of precise association between genetic and biochemical data. There is plenty of information, about 45,000 publications on mitochondrial structure and function and 2600 on mitochondrial diseases. However, we are in need of scientists that can combine the rigor of the clinical analytical biochemist with the specialized approach and inquisitiveness of the mitochondrial researcher. Therefore, discussion of methodological problems and standardization is especially important. Dr. Ernster provided an historical introduction and perspective of the exponential growth of information on mitochondrial diseases. This field grew from a single paper in 1959 to 2500 in 1996, and that now encompasses more than one hundred diseases including aging and chronic adult neurodegenerative diseases. He also discussed present work carried in his lab with CoQ deficiencies, presenting the pathways for the synthesis of CoQ and points where defects in the pathway can arise. He also discussed the vicious circle of mitochondrial DNA mutations and oxidative stress. Dr. C.P. Lee developed the important point, sometimes not easy to achieve, that only with isolated, coupled mitochondria, is it possible to start diagnosing mitochondrial defects with a certain degree of precision. If not done in such a way, confusion will result when attempting to correlate genotype with the biochemical lesion. She offered a precise example of a case where multiple defects in a patient were found only after exploring coupled mitochondria function not only with the oxygen electrode but also with spectral and stability studies of the components of the respiratory chain. Dr. Lee referred also to the risks of studying enzyme kinetics in homogenates, where side reactions can lead to erroneous results. Studies with homogenates requires the judicious use and titration with respiratory chain inhibitors to verify the absence of side reactions. Use of more than one assay method for the same enzymatic process is necessary to make sure that results are coherent. Dr. Trijbels discussed substrate transport defects, all nuclear genome-dependent effects. They cannot be studied unless isolated coupled mitochondria are used. Dr. Kunz presented microtechniques for the study of mitochondral function using single muscle fibers and introduced the use of computer imaging and fluorescence sorting as tools for diagnosis. While still highly specialized, his techniques offer an interesting and needed miniaturization approach to analytical procedures.