We studied the effect of a progestin (lynestrenol) on estrogen receptors (ER) and cathepsin D (cath-D) levels immunochemically in successive fine needle aspirates of benign breast disease. Fibrocystic disease was the main pathology (43 out of 47 patients). Thirty-one patients were treated with 10 mg of lynestrenol daily from the fifth to the twenty-fifth day of the menstrual cycle for 1 to 3 months. Sixteen untreated patients were used as controls. Lynestrenol significantly decreased the percentage of ER stained cells. This is in agreement with the antiestrogenic effect of progestin and, for the first time, indicates that in vivo progestin may decrease the stimulatory effect of estrogens on mammary cells by decreasing their estrogen receptor content. No effect of progestin on cath-D level was found throughout the whole population. However, this level varied more between aspirates of each patient in the treated group than in the control group, suggesting heterogeneity in patient responses to progestin. Since cath-D may have a role in carcinogenesis, clinical follow-up of these patients and more detailed studies are required to determine whether this progestin-challenge test has any value for detecting high risk mastopathies and for predicting effectiveness of treatment.