BACKGROUND There is no published data assessing whether higher mid luteal serum progesterone (P4) levels are associated with a higher cycle pregnancy rate (CPR) in controlled ovarian hyperstimulation (COH) with intrauterine insemination (IUI). OBJECTIVE To assess whether the mid luteal serum P4 level is predictive of pregnancy in COH with IUI. METHODS A retrospective cohort study of all women with unexplained, minimal endometriosis or mild male factor infertility who underwent COH with IUI between October 1999 and December 2000 at our department was analysed. The COH was achieved with follicle stimulating hormone injections. All cycles were triggered with human chorionic gonadotropin when at least one follicle > or =15 mm was visible on ultrasound and IUI performed the following day. A serum P4 and beta human chorionic gonadotropin level was measured at 7 and 14 days post-trigger, respectively. RESULTS There were 33 pregnancies in the 188 cycles analysed, giving a CPR of 18%. The median (range) mid luteal P4 level for all cycles was 51 nmol/L (1.8-234). This did not differ between the pregnant (55 nmol/L) and non-pregnant (50 nmol/L) cycles (P=0.282, Mann-Whitney U-test). There was also no difference in CPR between cohorts below or above the cut-off levels of 33 nmol/L (25th percentile) (13.3 vs 18.9%; P=0.39), 51 nmol/L (50th percentile) (16.0 vs 19.1%; P=0.57), or 69 nmol/L (75th percentile) (16.3 vs 21.3%; P=0.44), respectively. CONCLUSIONS Increased mid luteal serum P4 levels are not associated with a higher CPR in women undergoing COH with IUI. However, a low mid luteal P4 level < or =25 nmol/L may help predict treatment failure.