Hi there. I retired 3 years ago and don't know why I received a random copy of Fertility & Sterility in the mail this month but since it was here I looked through the Table off Contents and saw this article on Luteal Phase Deficiency[LPD]. . When I was a new Fellow at Duke in 1976 my 1st assignment was to summarize that clinic's experience with LPD. Little did I know or appreciate then that this disorder was/is very controversial; later I used to say that if you want to get a room full of REI's yelling at each other start a discussion on LPD. As it played out in my career I couldn't let go of this quirky disorder and ended up publishing a series of articles on its pathophysiology. The original diagnostic test [an endometrial biopsy] was subsequently proven to be too insensitive and a series of luteal progesterone levels was the only test that correlated with a low total luteal progesterone level [area under the curve; AUC] which was an accurate but impractical way to clinically identify LPD. In order to do those studies I did identify a small group of women with persistent LPD and showed that their brain [GnRH pulse generator] was the culprit in that the follicular phase LH pulse pattern was significantly different[faster] than normal. But LPD gradually became irrelevant in the clinical setting from about 1985 onward as it can occur randomly but rarely is persistent; is easily and adequately treated when empirical clomiphene citrate is prescribed; and is not a factor with super ovulation for IVF. Therefore, it was interesting to me to see LPD revisited in 2017. The criteria used here of a luteal phase of 11 days or less is valid based on our earlier studies as all our women with a short luteal phase had low AUC luteal progesterone levels. But the incidence of LPD has rarely been studied in a relatively large group of women attempting conception as was done here. This was a robust study group [n=284] and the women with LPD as defined by a short luteal phase were temporarily sub-fertile but not persistently so. This all fits with my final conclusions about the disorder: a. it is relatively rare b. usually not persistent c. on the spectrum of ovulatory disorders LPD is the mildest deviation from normal that can be caused by a host of common things like physical or psychological stress, weight fluctuations, and ovarian aging. It was a nice contribution by the authors to revisit LPD as I believe every REI physician needs to be aware of its existence as it will occasionally be present in the clinic setting.