Working at night and in shifts, as well as an unhealthy lifestyle, may increase the risk of breast cancer in nurses who therefore should frequently perform breast self-examination (BSE). The aim of the study was to investigate the performance of BSE among Polish nurses, its accuracy, sources of knowledge and skills, self-assessment of own competences and preparation to educate women about BSE. A descriptive, cross-sectional study was conducted in a group of female nurses (N=1,242). An anonymous, self-administrated questionnaire was used for data collection. To properly assess the BSE a four-item scale was made of the type single best answer multiple choice question. Each item referred to one agreed principle of performing BSE. Regularly BSE was performed by 56.1% nurses, 67.3% pre-menopause nurses 2-3 days after cessation of menstruation and 30.4% post-menopause women on a chosen day of the month. About 98% examined visually and by palpation; 58.9% did so in two positions. In the accuracy scale of BSE, the average number of points was 2.8 out of 4. All (4 points) or almost all (3 points) recommendations of accurate BSE were met by 61.4% of the nurses. Self-assessment of knowledge and BSE practical skills were considered as good or very good by 93.5% and 88.8% nurses, respectively. The self-evaluation of nurses' knowledge and BSE skills was significantly correlated with the result on the accuracy scale or this BSE. Many deficiencies concerning frequency, times and BSE techniques were revealed among Polish nurses. There is a discrepancy between the high self-assessment of competences and the accurate practice of BSE. Nurses' preparation in Poland in BSE is insufficient and requires improvement.