7% of 204 patients with metastatic breast cancer had hyperprolactinemia (greater than 1000 mIU/1 = 30.8 ng/ml) in the measurements of morning plasma prolactin basal levels. The incidence of hyperprolactinemia was significantly higher in patients with metastatic breast cancer than in 173 patients with non-metastatic breast cancer (p less than 0.001), in 151 patients with mastopathy (p = 0.01), in 63 patients with local (p = 0.001) and 56 patients with advanced solid tumors of different histology without prolactin stimulating medication (p = 0.001). After 5 measurements with a median interval of 2 months, elevated prolactin levels over 1000 mIU/l were found at least once in 35% of the patients with metastatic breast cancer. 93% of the women with hyperprolactinemic breast cancer were in progression at the time of the measurements, and none in remission; on the other hand, all patients with advanced breast cancer in remission had normal prolactin levels.