Plasma PRL, TSH, total and free T<sub>4</sub>, total and free T<sub>3</sub>, and 17β-estradiol were evaluated in 29 premenopausal women with well-documented fibrocystic disease of the breast and in 29 healthy matched controls. Plasma PRL and TSH dynamics after acute TRH injection (200 µg i.v.) were also determined. All hormonal measurements were performed in the follicular phase of the menstrual cycle. Neither patients nor controls showed any thyroid function impairment. Basal plasma levels of the examined hormones were in the normal range in both groups. When considering data pertinent to PRL and TSH secretory patterns after TRH stimulation, no difference was recorded between patients and controls for TSH secretion, evaluated in terms of maximum peak, net (Δ) and percent (Δ%) increase above the baseline level and integrated area of response. On the contrary, the response of PRL was significantly higher in patients than controls (maximum peak at 20min, mean ± SE, 119.9 ± 14.1 vs. 60.8 ± 5.5 ng/ml, p < 0.001; integrated area of response, 5,725 ± 908 vs. 3,243 ± 266 ng/ml/120 min, p < 0.01). The results are compatible with the view that, in most patients with fibrocystic disease of the breast, there are abnormalities in the control of PRL secretion, which lead to enhanced release of the hormone after stimulation. In such cases the control of TSH appears to be operating normally.