Background and purpose. Radiation therapy (RT) for breast cancer after conservative surgery can be life-saving but remains associated with significant late side effects, including lung fibrosis, detected by chest CT. Aim of this study was to assess whether lung ultrasound (LUS) may detect late lung fibrosis through the biomarker of B-lines. Materials and methods. We evaluated 30 women (median age 67 years, range 46-80 years) about 3-8 years after RT (follow up 38-101 months, median 58 months) for left (n = 12) or right (n = 18) breast cancer (stage 1, n = 24; stage 2, n = 6), treated with total dose 40.5 – 50.00 Gy with/without boost dose). In all, both treated and contralateral hemithorax were evaluated. LUS was performed and B-lines evaluated with a 28-region antero-lateral scan, from second to fifth intercostal spaces, along the mid-axillary, anterior axillary, mid-clavicular, and arasternal lines. In each space, the B-lines were counted from 0 = black lung to 10 = white lung. The sum of B-lines in all spaces generated the B-line score of each hemithorax. Results. Median B-line score was higher in the irradiated site than in the contralateral control hemithorax (9, 1st-3rd quartiles: 2-23 vs 3, 1st-3rd quartiles: 1-4; P < 0.05). In the treated hemithorax, higher mean lung doses ( > median value of 2.7 Gy) were associated with more B-lines when compared to lower doses (< 2.7 Gy): 9 vs 5, p < 0.001. Conclusion. RT in female breast cancer survivors is associated with increase in B-lines in the targeted hemithorax, likely due to lung fibrosis, and related to the lung mean dose. LUS can provide a simple “echo-marker” of lung fibrosis.