The lack of ovulatory cycles may be considered as a major problem for women seeking pregnancy. This is reflected by the fact that about 20 percent of couples visiting a fertility clinic with an unfulfilled wish to conceive present with anovulation. Clinical manifestation of anovulation is oligomenorrhea (intermenstrual period > 35 days) or amenorrhea (intermenstrual period > 6 months). Although ovulation may occur in oligomenorrhea, the longer the time period between menstruations the smaller the chance of that cycle being ovulatory. Classification of anovulatory patients may be performed using the criteria of the World Health Organization (WHO) as determined by Rowe et al. Criteria needed to classify patients are 1) serum prolactin in the normal range, 2) oligo- or amenorroea, and 3) serum concentrations of follicle-stimulating hormone (FSH) and estradiol (E2).In case of a hyperprolactinemia, a macro-prolactinoma should be ruled out by a scan (CT or MRI) of the sella turcica, and hyperprolactinemia should be treated with a dopamine-agonist. In case of normoprolactinemia or when the oligo- or amenorrea persists after correction of the hyperprolactinemia, these patients may be classified according to WHO criteria as follows: WHO1) ± 10% hypogonadotropic, hypoestrogenic status; WHO2) ± 80% normogonadotropic, normoestrogenic status; or WHO3) ± 10% hypergonadotropic, hypoestrogenic status. The vast majority of these patients, the WHO2 group, appear to be a very heterogeneous population in which – besides anovulation – obesity, biochemical, or clinical hyperandrogenism (alopecia, acne, or hirsutism) and insulin resistance play an important role.