Hyperprolactinemia is frequent in clinical endocrinology. Its commonest causes are, besides pregnancy and lactation, drugs, mainly involving the generally used psychopharmaca and the equally ubiquitously prescribed estrogens. The single most important cause is a pituitary tumor, the prolactinoma, but lesions of the hypothalamus or pituitary stalk, primary hypothyroidism, liver cirrhosis and chronic renal failure, among others, may also provoke hyperprolactinemia. The clinical features of hyperprolactinemia in women are mainly amenorrhea, or irregular menses, galactorrhea, hirsutism, infertility and loss of libido. In men loss of libido and/or impotence are the most important symptoms, accompanied by infertility. Macroadenoma, more frequently seen in men than in women, may cause tumor symptoms such as headache and ophthalmologic disorders (visual field loss). The main biochemical finding is hyperprolactinemia, which should be repeatedly checked. In general, high concentrations are mainly found in large adenomas, while microadenomas usually involve only mild hyperprolactinemia, though there are numerous exceptions. While dynamic tests of prolactin secretion have provided useful information about the pathophysiology of prolactin secretion, their use in routine clinical work is controversial and of limited value. As a routine neuroradiological examination, high resolution CT of the pituitary area is to be recommended. In all hyperprolactinemic patients with suspicion of macroadenoma, ophthalmologic evaluation of fundus and visual fields should be performed. Dopaminergic drugs such as bromocriptine rapidly reduce serum prolactin levels in hyperprolactinemic women and men with micro- or macroadenoma. With these drugs considerable tumor shrinkage is possible.(ABSTRACT TRUNCATED AT 250 WORDS)