Objective: Oocyte cryopreservation protocols have not been fully optimized yet. Our aims were to evaluate and compare the ultrastructure of human mature oocytes frozenthawed (F/T) after slow freezing and vitrification. Method: The oocytes, fixed at sampling (fresh controls) and after freeze/thawing, were processed for light and transmission electron microscopy (LM and TEM) observations. Results: By LM, both fresh and F/T oocytes were rounded cells surrounded by an intact zona pellucida (ZP) and containing uniformly distributed organelles. By TEM, numerous vacuoles were found in F/T oocytes after slow freezing. On the contrary, vacuoles were only occasionally detected in F/T oocytes after vitrification, and in fresh controls as well. Amount and density of cortical granules (CGs) appeared abnormally reduced in F/T oocytes, irrespective of the protocol applied. Conclusion: In conclusion, a) cryopreservation currently ensures a good overall preservation of the oocyte; b) however, vacuolization appears as a recurrent form of cell damage during slow freezing, whereas the quasi absence of vacuoles seems the most relevant marker of quality in vitrified oocytes; c) premature CG exocytosis -and the consequent hardening of the ZP -seems a non-specific, ubiquitous phenomenon occurring during freeze/thawing, suggesting the appropriateness of the use of ICSI as the preferred insemination method after cryopreservation.