BACKGROUND: The α-emitting radionuclide radium-223 (223Ra) is widely used for the treatment of bone metastasis in patients with castration-resistant prostate cancer. However, 223Ra decays into radon-219 (219Rn) which is a noble-gas isotope, and 219Rn may escape from patients treated with 223Ra via their respiration. In this study, we quantified the amount of 219Rn contained in the breath of patients treated with 223Ra to estimate its effect on the internal exposure dose of caregivers. METHODS: A total of 12 breath samples were collected using a breath collection bag from a total of six patients treated with 223RaCl2. Approximately 300 mL of exhaled breath was collected in a breath bag at 1 min and at 5 min after the start of 223RaCl2 administration. The contents of each bag were measured using an HPGe detector, and the amount of 219Rn was quantified based on the detection of the γ peak of 211Bi, which is a descendant nuclide of 219Rn, persisting in the breath bag. The effective dose to caregivers arising from the inhalation of 219Rn was estimated by referring to the scenario for the calculation of release criteria established for 131I therapy in Japan. RESULTS: A small peak for the 351-keV γ ray of 211Bi originating from the exhalation of 219Rn was observed. Using the observed γ peak of 211Bi, the average amounts of 219Rn per unit breath volume at 1 min and 5 min after the start of 223RaCl2 administration were calculated as 90 ± 56 Bq/mL and 28 ± 9 Bq/mL, respectively. The effective dose of 219Rn to caregivers was estimated to be 3.5 μSv per injection. CONCLUSIONS: The amount of 219Rn in the exhaled breath of patients treated with 223RaCl2 was quantitatively calculated using breath collection bags. The internal radiation exposure of caregivers from 219Rn in the exhaled breath of patients treated with 223RaCl2 is relatively small.