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Aims: In order to evaluate the pathogenesis of cleft-lip in relation to both the anatomical and structural anomalies of the mesenchymal tissues, the authors concluded that the presence of structural anomalies in the examined tissues could not explain the malformation, but might be a consequence of it. Delayed muscular development, asymmetrical distribution of the muscular fibres and their anomalous insertion suggest that the anatomical/functional loss clinically detectable in the orbicular muscle could be the result of a perinatal dysmorphological process rather than of a simple mesenchymal hypoplasia., Methods: Schendel et al. suggested that a metabolic defect in the mitochondrial function could cause a deficiency in cell migration and proliferation responsible for the malformation in question. To establish whether the pathogenesis of the cleft-lip is associated with an alteration in mitochondrial functionality, eight patients affected by unilateral cleft-lip were subjected to a biopsy of the orbicular muscle during the course of reparative surgery., Results: The results obtained showed: 1) a great variation in the size of muscle fibres; 2) the absence of ragged red fibres; 3) a normal oxidative function in the muscle fibres examined; 4) the absence of typologically significant groupings positive for myofibral ATPases. Furthermore, the morphology of the mitochondria was preserved in all cases and neither inclusions nor morphological or volumetric changes were detected., Conclusions: This preliminary data did not confirm the constant presence of mitochondrial pathology responsible for the malformation in question. In our opinion, the growth deficiency of the maxillary segment could be ascribed to the cicatrization of the surgical repair of the cleft-lip.