Ovarian stimulation for in-vitro fertilization (IVF) causes development of several cohorts of follicles. At the time of oocyte collection, oocytes are thus retrieved from a wide range of follicles of different sizes and developmental stages. A relationship between size of follicles and pregnancy rates has earlier been demonstrated. The aim of the present study was to compare fertilization, cleavage and pregnancy rates between oocytes retrieved from large and small follicles in conventional IVF and intracytoplasmic sperm injection (ICSI). A total of 200 conventional IVF patients and 175 ICSI patients underwent oocyte retrieval where oocytes from both large and small follicles were collected. A follicle with a volume of > or = 2 ml, corresponding to a follicular diameter > or = 16 mm as determined by ultrasound, was regarded as a large follicle. Only one cycle from each patient was included. Fertilization and cleavage rates were calculated per patient for oocytes from large and small follicles. The mean fertilization and cleavage rates for conventional IVF and ICSI cycles were calculated. Comparison of pregnancy rates was performed for patients receiving embryos derived from oocytes of only large or only small follicles. For conventional IVF patients, fertilization rates were 71.4 and 58.1% (P < 0.01, Wilcoxon paired test) for oocytes of large and small follicles respectively. The corresponding cleavage rates were 95.4 and 93.9% respectively. The pregnancy rate for the two groups was 47% (60/127) and 15% (2/13) (P