Hyperstimulation protocols for IVF require critical timing of hCG administration to complete follicular maturation. We studied 170 IVF cycles in 142 patients and retrospectively compared ultrasound and endocrine parameters in pregnancy cycles and spontaneous LH surge cycles with nonpregnancy, non-LH surge cycles. Twenty-six of the 170 cycles resulted in a pregnancy (15.2%). Oocytes were recovered from 27 of 31 cycles in which a spontaneous LH surge occurred, and 9 of these cycles produced a pregnancy (33%). Follicle number and serum E2 on the day of ultrasound (cycle day 12 or 13) was lower in the LH surge group and pregnancy group than in the nonpregnant, non-LH surge group. In contrast, on the day of hCG administration or LH surge, serum E2 concentration per follicle greater than 1 cm in diameter was higher in the combined LH surge/pregnancy group (1219 pmol/l) than in the nonpregnancy/non-LH surge group (932 pmol/l). Our data suggest that it may be possible to individualize hCG administration at midcycle by determining the number of follicles greater than 1 cm by ultrasound on cycle day 12 or 13 and giving hCG when serum E2 levels reach 1100 to 1200 pmol/l per follicle.