The objective of this study is to compare the pregnancy and live birth rates of a natural cycle in vitro fertilization (IVF) combined with in vitro maturation (IVM) treatment (natural cycle IVF/M) by the presence or absence of mature oocytes retrieved. Infertile women were divided into two groups: (A) patients with mature oocytes found at retrieval and (B) patients with only immature oocytes at retrieval. Patients of group A were further divided into three subgroups: (A1) mature oocytes retrieved from both the leading and the small follicles, (A2) mature oocytes retrieved from the leading follicles only, and (A3) mature oocytes retrieved from the small follicles only. Pregnancy and implantation rates were compared. The results indicate that the clinical pregnancy rates were 40.1% (126/314) and 34.5% (19/55) for groups A and B, respectively. There were no differences in pregnancy rates among the subgroups: A1=44.0% (66/150), A2=34.9% (30/86), and A3=38.5% (30/78). In addition there were no differences in implantation rates among the groups (16.2% =139/859, 15.0% =22/147, 16.8% =69/410, 14.7% =34/232, and 16.6% =36/217, respectively). However, the live birth and miscarriage rates were significantly different between the group A and group B (29.6% =93/314 vs. 16.4% =9/55 and 26.2% =32/126 vs. 52.6% =10/19, respectively). In conclusion, for natural cycle IVF/M treatment, although the clinical pregnancy rates are not different regarding the retrieval of mature oocytes or the time of the egg retrieval, the live birth rate is higher (P < 0.05) when the mature oocytes are obtained at the time of the egg retrieval.