In application of mammography and aiming biopsy, synchronous carcinoma of contralateral breast may be detected in 2.4% of cases, on the average, 3 years earlier than by clinical examination only. Increased proliferation of contralateral breast tissue (a factor contributing to tumor development) is not a pathognomonic symptom since proliferation rate in ductal and lobular tissues decreases with age and in menopause. At least half metachronous carcinomas of the second breast should be considered synchronous ones overlooked in the course of primary treatment.