Objective: To examine whether synchronized administration of hCG at the onset of the endogenous LH rise promotes successful IVF. Design: A prospective randomized study. Setting: In vitro fertilization program at a university hospital. Patients: A total of 208 IVF cycles in 148 patients. Interventions: Serum LH concentrations were measured daily and hMG was administered daily. Independent of follicle size and E(2) concentration, hCG was administered as soon as the LH concentration exceeded the J level, defined as the minimum value + (the day 3 value - the minimum value) X 1/3(J group). Alternatively, hCG was administered when the serum LH concentration turned to increase but was still less than the J level, or 1 day after the serum LH concentration exceeded the J level (non-J group). Results: The rates of total and ongoing pregnancy per cycle were significantly higher in the J group (35.6% and 26.0%, respectively, n = 104) than in the non-J group (21.2% and 12.5%, respectively, n = 104). Pregnancies in the J group were achieved over a wide range of dominant follicle diameters (13 to 25 mm), E(2) levels (198 to 1,700 pg/mL; conversion factor to SI units, 3.671), and E(2) level per follicle greater than or equal to 12 mm (24 to 225 pg/mL per follicle) recorded on the day of hCG administration. Conclusion: Synchronized administration of hCG in accordance with endogenous LH rises produces a high rate of pregnancy in IVF.