In 193 patients suffering from fibrocystic breast disease, basal serum prolactin concentrations were determined and compared to serum prolactin levels in 193 healthy women. In 45 additional patients and 23 healthy control subjects, a thyrotropin-releasing hormone (TRH) stimulation test was performed. The response to TRH in seven healthy female volunteers and in one patient with fibrocystic breast disease, was correlated with the mean serum prolactin levels over 24 hours. Serum prolactin levels were above normal in 45.6% of the patients and in 21.2% of the control subjects. Mean values of the two groups were significantly different (P less than .001). The maximum prolactin response to TRH stimulation was significantly higher in the study patients than in the control subjects (P less than .001). The TRH-stimulated prolactin response correlated positively with the mean 24-hour level (P less than .01, r = 0.8705). These results indicate that a high proportion of patients with fibrocystic breast disease exhibit increased daily prolactin secretion.