Objective. To investigate the eff iciency and safety of monotherapy with the transdermal micronized progesterone Progestogel for diffuse fibrocystic mastopathy. Materials and methods. The observational study enrolled 722 patients aged 37.1±0.3 years with preserved menstrual function and diagnosed diffuse fibrocystic mastopathy. The observation period consisted of a continuous medication usage period about 3 months (3 cycles), or, if necessary, a second course of as many as 6 months (6 cycles). The main criterion for the efficiency of the therapy was to estimate the number of masses, as evidenced by ultrasonography. During the study, the investigators made breast ultrasound and radiographic examinations, Breast Imaging-Reporting and Data System (BI-RADS) assessment, recorded pain intensity with a visual analogue scale (VAS), and assessed a patient’s health status according to the Clinical Global Impression Scale (CGI). Results. The clinical breast assessment throughout the investigation, by using the main effi