Basic problems of the group of hereditary mitochondrial beta oxidation (BOX) disorders are presented with evaluation of the role of pathologists in the diagnostic process. The disorders manifest themselves clinically as usual by recurrent Reye-like episodes (acute hepatopathy and encephalopathy) typically in low age levels. Integral part of the clinical picture is often a myopathic symptomatology which at some cases may display even persisting character. The findings at the tissue level are dominated mostly by steatosis of the organ set with a high beta oxidation level (liver, heart, skeletal muscle, kidney) and toxic effects of the intermediate metabolic products of the derranged beta oxidation process. So far eighteen enzyme defects have been described affecting either transport of fatty acids across the mitochondrial membranes or their oxidative degradation at various levels, pointing to an absolute dependence of the final diagnosis on biochemical analysis. Pathologist's conclusion in cases dying without diagnosis is limited to suspicion of a BOX disorder only. However, pathologists can contribute significantly to unraveling and specification of the underlying BOX defect by collecting adequate samples of body fluids and of unfixed organs. Nevertheless, the validity of these samples, even if widely recognized, is limited. The best approach is to provide samples enabling to perform biochemical evaluation of the whole BOX process in integral mitochondrias. This requirement is fulfilled solely by establishing fibroblast tissue culture post mortem.