This study compared the embryological characteristics and clinical outcome of in-vitro maturation (IVM) treatment cycles with and without in-vivo matured oocytes collected following human chorionic gonadotrophin (HCG) priming. The patients were administered 10,000 IU of HCG subcutaneously when endometrial thickness reached > or =6 mm and oocyte collection was performed 35-36 h after HCG administration. The clinical outcome and embryological aspects were analysed between IVM cycles with (group 1) and without (group 2) in-vivo matured oocytes. In group 1, three (range 1-12) in-vivo matured oocytes per patient were retrieved on average. The number of good quality embryos derived from in-vivo matured oocytes in group 1 was significantly higher than those derived from in-vitro matured oocytes in group 1 and group 2 (P < 0.05). However, there was no difference between the number of good quality embryos produced from in-vitro matured oocytes in the two groups. There were 12 clinical pregnancies (40.0%) in group 1, and seven pregnancies (23.3%) in group 2. These results suggest that IVM cycles with in-vivo matured oocytes resulted in a good clinical pregnancy rate, which could be explained by the superior quality of embryos derived from the in-vivo matured oocytes.