BackgroundThis subgroup analysis of the Cancer-VTE Registry, a nationwide, large-scale, multicenter observational study with a 1-year follow-up, assessed real-world data on venous thromboembolism (VTE) among Japanese patients with breast cancer.MethodsPatients with stage II–IV pretreatment breast cancer screened for VTE at enrollment were included. During the 1-year follow-up period, incidences of VTE, bleeding, and all-cause death, and background factors associated with VTE risk were examined.ResultsOf 9,630 patients in the Cancer-VTE Registry analysis set, 993 (10.3%) had breast cancer (973 [98.0%] did not have and 20 [2.0%] had VTE at baseline). The mean age was 58.4 years, 73.4% of patients had stage II cancer, and 94.8% had an Eastern Cooperative Oncology Group performance status (ECOG PS) of 0. Risk factors for VTE at baseline by univariable analysis were age ≥ 65 years, ECOG PS of 2, VTE history, and D-dimer > 1.2 μg/mL. During follow-up, the incidence of symptomatic VTE was 0.4%; incidental VTE requiring treatment, 0.1%; composite VTE (symptomatic VTE and incidental VTE requiring treatment), 0.5%; bleeding, 0.2%; cerebral infarction/transient ischemic attack/systemic embolic event, 0.2%; and all-cause death, 2.1%. One patient with symptomatic VTE developed pulmonary embolism (PE) and died. Incidences of VTE and all-cause death were higher in patients with VTE vs without VTE at baseline.ConclusionsIn Japanese patients with breast cancer, VTE screening before initiating cancer treatment revealed a 2.0% prevalence of VTE. During follow-up, one patient had a fatal outcome due to PE, but the incidences of VTE were low.Clinical trial registrationUMIN000024942; UMIN Clinical Trials Registry:https://www.umin.ac.jp/ctr/.Supplementary InformationThe online version contains supplementary material available at 10.1007/s12282-023-01452-7.