Phylloid fibroadenoma of the mammary gland is a comparatively rare lesion--0.3% of all breast affections, its clinical course is generally known to be characterized by both the possibility of recurrences development after non-radical treatment and its transition into mammary gland sarcoma. The authors report the data on 56 cases of phylloid fibroadenoma of the mammary gland treated at the N.N. Petrov Research Institute of Oncology of the USSR Ministry of Health during the period of 1926 to 1973. Fifty one of these patients showed benign tumors, 5--malignant tumors. The clinical diagnosis of the tumor is rather complicated (the correct preoperative diagnosis was established only in 27.5% of cases). The use of non-contrast mammography and trepanbiopsy failed to appreciably improve the diagnosis. The differential diagnosis between the benign and malignant forms seems to be mostly difficult. Only surgery is advocated. The extent of surgical intervention depends on morphological features of the tumor, its size, rate of growth and the breast size. Among benign forms recurrences were noted in three patients (6.3%). Of 5 patients with malignant phylloid fibroadenoma two patients died due to metastases during two postoperative years.