Piomboni P1, Bruni E, Capitani S, Gambera L, Moretti E, La Marca A, De Leo V, Baccetti B. Abstract Seventeen sperm samples were evaluated by transmission electron microscopy (TEM) before and after swim-up separation. DNA-fragmentation was tested by terminal d-UTP nick end labeling (TUNEL) in unselected and selected semen samples, and the results were analyzed in relation to sperm ultrastructural characteristics detected by TEM. A significant improvement in mean numbers and percentages of structurally normal sperm was observed after swim-up selection, corresponding to a significant decrease in the percentage of necrotic and apoptotic sperm, while the percentage of sperm with immature nuclei did not change significantly. TUNEL indicated a significant decrease in chromatin-fragmented sperm after swim-up. Swim up selection based on sperm motility excludes many sperm with ultrastructural evidence of necrosis (absent or reacted acrosome, disrupted chromatin, broken plasma membrane) and apoptosis (misshapen nuclei with marginated chromatin), as confirmed by TUNEL analysis. Nevertheless, immature sperm with elliptical or roundish nuclei, misshapen acrosomes and uncondensed chromatin remain part of fertilizing pool.