Perimenopause, rather than a time of declining estrogen, is characterized by three major hormonal changes that may begin in regularly menstruating women in their mid-thirties: erratically higher estradiol levels, decreased progesterone levels (in normally ovulatory, short luteal phase or anovulatory cycles), and disturbed ovarian-pituitary-hypothalamic feedback relationships. Recent data show that approximately a third of all perimenopausal cycles have a major surge in estradiol occurring de novo during the luteal phase. This phenomenon, named "luteal out of phase (LOOP)" event, may explain a large proportion of symptoms and signs for symptomatic perimenopausal women. Large urinary hormone data-sets from women studied yearly over a number of years in the Study of Women Across the Nation (SWAN) and in the Tremin data will eventually provide a more clear prospective understanding of within-woman hormonal changes. Predicting menopause proximity with FSH or Inhibin B levels is documented to be ineffective. Anti-Mullerian hormone levels may prove predictive. Finally, there is an urgent need to change perimenopause understandings, language and therapies used for midlife women's symptoms to reflect these hormonal changes.