In several cases of asthenoteratozoospermia, electron microscopic investigation displayed immature sperm forms, morphological apoptotic patterns of spermatozoa and many cytoplasmic conglomerates with fragments of the sperm. In these patients, TUNEL assay showed a high percentage of spermatozoa with nuclear DNA fragmentation. Moreover, thickened and deformed midpieces were observed which contained supernumerary and redundant mitochondria with normal oxidoreductive capability and normal membrane potential. In these cases a high percentage of spermatozoa with normal DeltaPsim was detected. Nevertheless, a subpopulation of patients was found with an abnormal ultrastructure of sperm mitochondria and with a low percentage of spermatozoa with normal DeltaPsim. These findings indicate that low motility of spermatozoa may be related to abnormal morphogenesis of the midpiece containing functional mitochondria and that this may be a possible consequence of an apoptotic mechanism. Furthermore, our results show that asthenoteratozoospermia may result from dysfunction of sperm mitochondria and/or with alternations of the structures involved in sperm motility, i.e. the dense outer fibres, the fibrous sheath and the axoneme.