The cytometry of 545 oocytes was evaluated during subzonal insemination (SUZI; 85 attempts), on day 0 (egg retrieval and SUZI), day 1 and day 2(embryo transfer). On day 0, the egg and oolemma diameters (mean ± SD) were 164.0 ± 19.6 μm and 114.2±16.8 μ5m respectively.The zona thickness was 17.8± 13.4 μm and correlated with the oolemma diameter(r = 0.24, p < 0.001). The fertilisation rate was significantly lower for the smaller oocytes (less than 108 μm diameter) compared with the larger oocytes (over 108μm) (9.8% vs 21.2% respectively; p < 0.05). These was little variation in oocyte diameter according to nuclear status. However, oocyte diameter increased significantly between day 0 and day 1 (p < 0.001) for both fertilised and unfertilised oocytes. Six different indications for SUZI were investigated in detail: three with non-specific (normal and subnormal sperm with in vitro fertilization failure, oligoasthenospermia) and three with specific sperm defects (flagellar dyskinesia, absence of outer dynein arms, antisperm antibodies). Oocytes from the non-specific defect groups had significantly smaller diameters than the others (p < 0.05). The mean fertilisation rate was related to the mean oolemma diameter for the groups with non-specific sperm defects and the group lacking dynein arms (LODA) (r = 0.91, p < 0.05). Eggs from the groups of patients with LODA and those with antisperm antibodies had thicker zona pellucida than others (p < 0.05). These findings suggest that in addition to nuclear criteria of maturity, the growth of oocytes is an important factor for fertilising ability. Insufficient development of the ooplasm may contribute to fertilisation failure, particularly when sperm with functional defects are used. In contrast, a thick zona pellucida may prevent sperm with specific anomalies such as LODA or antisperm antibodies from penetrating into the perivitelline space.