OBJECTIVE To investigate whether mid-luteal serum progesterone (P4) exhibits significant fluctuations during a 12-h daytime period in women undergoing in vitro fertilization (IVF) and to explore whether the extent of these fluctuations could impact the interpretation of luteal progesterone levels in a clinical setting. METHODS Explorative pilot study. METHODS Public hospital-based fertility unit. METHODS Ten women undergoing IVF treatment. METHODS Seven days after oocyte pick-up, patients underwent frequent repeated blood sampling (every 60 min for 12 h and during two of these hours, every 15 min). Serum samples were analyzed for progesterone, estradiol, and luteinizing hormone (LH). METHODS Daytime fluctuations in s-progesterone and s-estradiol. RESULTS There was a significant positive correlation between median P4 levels and the magnitude of P4 variations-women with median P4 < 60 nmol/l had clinically stable P4 levels throughout the day, while patients with median P4 > 250 nmol/l exhibited periodic P4 peaks of several hundred nanomoles per liter. These endogenous P4 fluctuations were observed irrespective of the type of stimulation protocol or mode of triggering of final oocyte maturation and despite the fact that LH was under the detection limit at the time of measurement. Simultaneously, large fluctuations were seen in s-estradiol. CONCLUSIONS Monitoring of early to mid-luteal P4 levels in IVF cycles may be valuable in the planning of individualized luteal phase support in the attempt to increase reproductive outcomes. The prerequisite for luteal phase monitoring is, however, that the validity of a single measured P4 value is reliable. We show for the first time, that a single P4 measurement in the low progesterone patient quite accurately reflects the corpus luteum function and that the measurement can be used to detect IVF patients with a need of additional exogenous luteal P4 administration.