Another functional disturbance that should be addressed is the syndrome of the luteinized but unruptured follicle (LUF syn­drome). In LUF syndrome, rupture of the preovulatory follicle does not occur despite luteinization. Progesterone production may be deficient in LUF syndrome or may be within normal limits. The postovulatory parameters may also be essentially normal. This can mimic a “presumed ovulatory” cycle, making it very difficult to diagnose LUF syndrome. The syndrome can have various causes: a deficient LH surge, an absence of pre­ovulatory progesterone secretion, primary oocyte abnormali­ties, and changes in progesterone synthesis or other mediators leading to rupture of the follicle. The condition was initially di­agnosed laparoscopically, and later its diagnosis was aided by detecting low concentrations of ovarian steroids in the cul-de­sac fluid. As ultrasound techniques are refined, it is reasonable to expect that new discoveries on this rare syndrome will be forthcoming.