In a study, conducted by two clinics in Berlin and Hamburg, specializing in reproductive endocrinology, the anamnestic, clinical, and laboratory data of 170 oligomenorrheic patients (menstrual intervals between 35 and 90 days) were evaluated in order to determine the frequency of possible causes of oligomenorrhea. Pathological hormone levels were found in two thirds of all patients. The order of frequency of abnormal hormone levels was as follows: hyperandrogenemia (testosterone and/or DHEA-sulfate) in 41.8%, hyperprolactinemia in 25.9%, abnormal thyroid function (TSH and/or TRH-induced TSH) in 21.7%, and hypergonadotropic FSH levels in 3.5% of all patients. There was an overlap of between two or more pathological conditions in one third of all patients. This study confirms results of a previous study in amenorrheic patients (Moltz et al., 1987 - see reference list), documenting hyperandrogenemia as the most frequent abnormality found in this group, followed by hyperprolactinemia. As can be expected, the percentage of women with no discernible abnormality was higher in oligomenorrheic patients when compared with the amenorrheic group (32.3% vs 7.7%). Furthermore, overweight patients were overrepresented in the oligomenorrheic group, while underweight patients were seen more frequently in the amenorrheic group. In view of these results of our study we recommend a detailed diagnostic follow-up in all younger patients with ovarian disorders who need to preserve their reproductive potential. This follow-up should include hyperprolactinemia, hypo-/hyperthyroidism, hyperandrogenemic and hypoestrogenemic states and exclusion of primary ovarian failure. In contrast to recommendations of WHO, issued in 1976, such diagnostic work allows an etiology oriented therapy decision and a therapy risk assessment in subgroups of patients, such as hyperandrogenemic patients, who receive clomiphene or gonadotropin treatment. Furthermore, it permits prophylactic considerations, for prevention of hirsutism and polycystic ovarian disease, struma and osteoporosis prophylaxis.