Many institutions worldwide currently deliver left breast radiotherapy in free breathing mode, mostly due to the unavailability of a Deep Inspiration Breath Hold technique (DIBH). This study aims at quantifying the error in dose delivery (compared to treatment plan) due to respiratory motion in free breathing irradiation of left breast or chest wall. Since subfields often consist in small, fine-tuned, highly targeted fields, slight intrafractional target motion may compromise their subtle benefit. Thus we analyzed the respiratory motion effect on target dose coverage, dose homogeneity and left lung dose. Treatment plans for twenty left breast or chest wall cancer patients previously treated at our center were retrieved and retrospectively planned with the introduction of an appropriate shift in isocenter location to simulate free breathing target motion. No clinically significant dosimetric changes were found in all twenty cases when breathing motion was accounted for. Changes in target dose coverage (V95%), in target maximum dose (D2%) and in V20Gy lung dose were respectively less than 1.5%, 0.3% and 2.6%. The findings suggest that breast irradiation in free breathing mode does not undermine the dosimetric merits of the field-in-field technique and does not produce clinically significant dosimetric differences in dose delivery for target and lung compared to plan. •Breathing has a negligible effect on dose delivery accuracy in breast cancer radiotherapy.•Free breathing during breast radiotherapy treatment does not invalidate the use of field-in-field technique.•The dosimetric error from free breathing breast motion during radiotherapy treatment is clinically negligible.