Premature LH and progesterone surges are associated with different factors and hormonal modulators. The aim of the present study was (i) to investigate the clinical and laboratory factors and (ii) to highlight the importance of different stimulation protocols in associated premature LH and progesterone surges in intrauterine insemination (IUI) cycles. The study involved a retrospective investigation of 75 patients undergoing IUI for infertility treatment (135 IUI cycles) between 1996 and 2000, with initial serum LH concentrations >10 mIU/ml during ovarian stimulation. Ultrasound characteristics, follicular sizes, serum oestradiol, progesterone and LH concentrations and ovarian stimulation protocols were measured. There was a wide range of oestradiol serum concentrations (93-2245 pg/ml) and follicular size (12-25 mm). In 49.6% of cycles, the dominant follicle was <16.5 mm. Patients with >2 follicles measuring <15 mm had higher oestradiol serum concentrations (P = 0.008). Multiple regression analyses revealed no association between these variables and premature LH/progesterone surge. In conclusion, LH/progesterone surges cannot be predicted utilizing clinical parameters normally employed, e.g. ultrasound serum oestradiol assay or ovarian stimulation protocol. Patients with follicles >14 mm or more and with high numbers of small follicles and high oestradiol are at risk of a spontaneous LH surge. These variables can be used to time the administration of GnRH antagonist administration until better predictive factors are demonstrated.