BACKGROUND: The negative impact of rising progesterone levels on pregnancy rates is well known, but data on mature oocyte yield are conflicting. We examined whether delaying the oocyte maturation trigger in IVF/ICSI affected the number of mature oocytes and investigated the potential influence of serum progesterone levels in this process. METHODS: Between January 31, 2011, and December 31, 2011, 262 consecutive patients were monitored using ultrasound plus hormonal evaluation. Those with > =3 follicles with a mean diameter of > =18 mm were divided into 2 groups depending on their serum progesterone levels. In cases with a progesterone level < = 1 ng/ml, which was observed in 59 patients, 30-50% of their total number of follicles (only counting those larger than 10 mm) were at least 18 mm in diameter. These patients were randomised into 2 groups: in one group, final oocyte maturation was triggered the same day; for the other, maturation was triggered 24 hours later. Seventy-two patients with progesterone levels > 1 ng/ml were randomised in the same manner, irrespective of the percentage of larger follicles (> = 18 mm). The number of metaphase II oocytes was our primary outcome variable. Because some patients were included more than once, correction for duplicate patients was performed. RESULTS: In the study arm with low progesterone (<= 1 ng/ml), the mean number of metaphase II oocytes (+/-SD) was 10.29 (+/-6.35) in the group with delayed administration of the oocyte maturation trigger versus 7.64 (+/-3.26) in the control group. After adjusting for age, the mean difference was 2.41 (95% CI: 0.22-4.61; p = 0.031). In the study arm with elevated progesterone (>1 ng/ml), the mean numbers of metaphase II oocytes (+/-SD) were 11.81 (+/-9.91) and 12.03 (+/-7.09) for the delayed and control groups, respectively. After adjusting for PCOS (polycystic ovary syndrome) and female pathology, the mean difference was -0.44 (95% CI: -3.65-2.78; p = 0.79). CONCLUSIONS: Delaying oocyte maturation in patients with low progesterone levels yields greater numbers of mature oocytes.