In 108 patients with nonpuerperal mastitis (inflammatory symptoms of duct ectasia), serum prolactin levels were determined before, during, and after treatment. Twenty-nine patients (26.8%) exhibited transiently elevated prolactin levels during the period of inflammation (mean +/- SD level, 42 +/- 22 micrograms/L) that returned to normal within 4 weeks. Twenty-two patients (20.4%) presented with more severe hyperprolactinemia (78 +/- 56 micrograms/L), and 15 were found to have pituitary microadenomas. In 11 cases, mastitis was the first symptom of hyperprolactinemia. A second group of 83 patients with known hyperprolactinemia and 83 normoprolactinemic controls were interviewed with regard to recent symptoms or past treatment of nonpuerperal mastitis. Sixteen hyperprolactinemic women (19.3%) and none of the controls reported that they had experienced mastitis. We conclude that duct ectasia is due in part to increased prolactin secretion. Thus, nonpuerperal mastitis may be a symptom of hyperprolactinemia. On the other hand, nonpuerperal mastitis may induce transient hyperprolactinemia (neurogenic hyperprolactinemia) of about 3 weeks' duration that is less pronounced than central hyperprolactinemia.