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Background: We assessed the morphologies of meiotic spindles in oocytes that failed to fertilize following intracytoplasmic sperm injection (ICSI) and identified factors contributing to failed fertilization., Methods: A total of 225 unfertilized oocytes were collected after ICSI. Oocytes were fixed and stained for tubulin and chromosomes. Meiotic spindle morphologies, chromosome alignment, and sperm nuclear decondensation were assessed to identify contributing factors to fertilization failure. We identified relationships between several factors and both abnormal spindle morphologies and sperm nuclear decondensation in oocytes that failed to fertilize., Results: Three causes for unfertilized oocytes after ICSI were identified: (I) the absence of a sperm nucleus in the ooplasm; (II) failed oocyte activation; and (III) defects in pronucleus formation or migration. The rate of disarranged polar spindles in oocytes collected from women older than 35 years (73.3%; 33/45 oocytes) was significantly higher than that of those collected from women 35 years and younger (50.4%; 68/135 oocytes; odds ratio [OR]: 2.71, 95% confidence interval [CI]: 1.29-5.69, p = 0.009). The proportion of unfertilized oocytes with abnormal spindles and chromosome misalignment was significantly higher in oocytes collected from women older than 35 years than those from women 35 years and younger (62.2% vs. 41.5%, p = 0.016). The proportion of partially decondensed chromatin in the abnormal sperm morphology group was significantly higher than in the normal sperm morphology group (66.7% versus 52.9%, OR: 1.78, 95% CI: 1.01-3.11, p = 0.044)., Conclusions: The main contributor to the failure of oocytes to fertilize after ICSI is failed oocyte activation. The ICSI technique used, the maternal age, and sperm morphology are also contributing factors in fertilization failure after ICSI.