Thirty women undergoing in-vitro fertilization or intracytoplasmic sperm injection considered to be at high risk of ovarian hyperstimulation syndrome (OHSS) were randomly allocated to have early unilateral follicular aspiration (EUFA) (group 1) or coasting (group 2) when the serum oestradiol concentration was >6000 pg/ml and there were more than 15 follicles each of >/=18 mm diameter in each ovary. EUFA was performed in group 1 at 10-12 h after the human chorionic gonadotrophin (HCG) trigger injection and human menopausal gonadotrophin (HMG) were withheld for 4.9 +/- 1.6 days until serum oestradiol concentrations fell below 3000 pg/ml when HCG was administered. The mean total dose and duration of administration of HMG were similar in groups 1 and 2 (48.3 +/- 17.4 and 50.2 +/- 16.5 ampoules; 13.7 +/- 2.2 and 14.1 +/- 3.2 days respectively). The mean serum oestradiol concentrations (9911 pg/ml versus 10 055 pg/ml) and number of follicles (43.3 versus 41.4) seen in both ovaries on the day of HCG administration in group 1 and on the day coasting was commenced in group 2 were also similar. After coasting, the mean serum oestradiol concentration on the day of HCG administration in group 2 was lower than in group 1 (1410 pg/ml versus 9911 pg/ml; P < 0.001). The mean serum progesterone concentrations on the day of HCG administration in both groups were similar, and fell in all women in group 2. The mean number of oocytes retrieved and percentage of oocytes retrieved per follicle punctured was significantly higher in group 1 (15.4 +/- 2.1 versus 9.6 +/- 3.2, P < 0.001; 91.4 +/- 4.4% versus 28.3 +/- 3.7%, P < 0.001 respectively). The fertilization and embryo cleavage rates were similar in both groups. Clinical pregnancy was diagnosed in 6/15 (40%) patients in group 1 and in 5/15 (33%) patients in group 2, while four women in group 1 and three in group 2 developed severe OHSS.