This prospective study was undertaken to establish the correlation between the follicular diameter, oocyte recovery, metaphase II (MII) oocyte, and fertilization rate in intracytoplasmic sperm injection (ICSI) programs. Thirty one patients undergoing 31 ICSI cycles from August 1998 to January 1999 at King Chulalongkorn Memorial Hospital were studied. The patients were stimulated with either short or long GnRHa portocol. During transvaginal aspiration of follicles, follicular diameters were measured. The maturation stage of oocytes and fertilization rate were evaluated. Four hundred and seventy eight follicles were measured and classified into 3 groups, group A (<10 mm), group B (10-14 mm) and group C (>14 mm). The oocyte recovery rate and MII oocytes were different between groups A, B, and C but the fertilization rate was not different. In conclusion, the follicular diameter correlated with oocyte recovery and oocyte maturation but not with the fertilization rate in ICSI programs. The quality of embryos was lower in oocytes from small follicles.