Objective: Since little is known about the actual incidence and fate of vacuoles at different stages of development this preliminary study was set up to accurately measure vacuoles and track them to day 5. Design: Prospective study. Setting: Women’s General Hospital in Austria. Patient(s): A total of 223 consecutive IVF and intracytoplasmic sperm injection (ICSI) cycles (206 patients). Intervention(s): Accurate measurement of vacuoles. Affected gametes and embryos were cultured individually and tracked until day 5. Main Outcome Measure(s): Size and number of vacuoles, fertilization rate, blastocyst formation rate, blastocyst quality. Result(s): There was a significant relationship between size of the vacuole (cutoff value 14 μ m) and fertilization (P< .05). At zygote stage the incidence of vacuoles was higher (P< .01) in ICSI (11.6% ) than in IVF (5.3% ). Only 32.2% of affected ICSI- embryos reached blastocyst stage on day 5 compared with 53.0% of the normal ones (P< .001). In terms of blastocyst formation vacuolization on day 4 (P< .001) turned out to be the most severe one. At blastocyst stage inner cell mass was affected less frequently than the trophectoderm (P< .05). Conclusion(s): Three types of vacuoles could be identified: (1) those already present at oocyte collection, which develop during maturation (day 0); (2) those artificially created by ICSI (day 1); and (3) those accompanied with developmental arrest (day 4). The later that vacuoles arose, the more detrimental their effect on blastocyst formation. Objective: Since little is known about the actual incidence and fate of vacuoles at different stages of development this preliminary study was set up to accurately measure vacuoles and track them to day 5. Design: Prospective study. Setting: Women’s General Hospital in Austria. Patient (s): A total of 223 consecutive IVF and intracytoplasmic sperm injection (ICSI) cycles (206 patients). Intervention (s): Accurate measurement of vacuoles. Affected gametes and embryos were cultured individually and tracked until day 5. Main Outcome Measure ( (s): There was a significant relationship between size of the vacuole (cutoff value 14 μm) and fertilization (P <.05). Atzygote stage the incidence of vacuoles was higher (P <.01) in ICSI (11.6%) than in IVF (5.3%). Only 32.2% of affected ICSI- embryos reached blastocyst stage on day 5 compared with 53.0% of the normal ones (P <.001). In terms o f blastocyst formation vacuolization on day 4 (P <.001) turned out to be the most severe one. At blastocyst stage inner cell mass was affected less frequently than the trophectoderm (P <.05). Conclusion (s): Three types of those already present at oocyte collection, which develop during maturation (day 0); (2) those artificially created by ICSI (day 1); and (3) those accompanied with developmental arrest (day 4) . The later that vacuoles arose, the more detrimental their effect on blastocyst formation.