The ultrasound detection of corpus luteum in the midluteal phase of ovulatory cycles was associated with significantly higher levels of plasma progesterone, opposed to those patients where a corpus luteum could not be identified. The hormone level was directly proportional no the number of corpora lutea seen. However, only 33.3% o patients with no corpus luteum visible were found to have suboptimal plasma progesterone levels (< 10 ng/ml).