AIMS: Cancer treatment-related cardiovascular toxicity (CTR-CVT) is a growing concern in patients undergoing anticancer therapy. The Heart Failure Association (HFA) and International Cardio-Oncology Society (ICOS) risk assessment tools have been proposed for the baseline cardiovascular (CV) risk stratification of patients with cancer. This study investigated the incidence of CV adverse events in clinical practice, also using the HFA-ICOS risk tool. METHODS AND RESULTS: This single-centre, prospective, observational study was conducted at Kurume University Hospital from October 2016 to August 2021, including patients aged ≥20 years with haematologic malignancies or breast cancer who were receiving anticancer agents. Cardiovascular assessments were performed at enrolment and every 6 months until August 2021, with additional assessments for suspected CV adverse events. The primary endpoint was common terminology criteria for adverse events v4.0 Grade ≥2, and the secondary endpoints were all-cause and CV deaths. Of the enrolled 486 patients, CV adverse events occurred in 24.5, 15.8, 38.1, and 18.0% of patients with leukaemia, malignant lymphoma, multiple myeloma, and breast cancer, respectively. Patients at high or very high risk had a significantly higher incidence of CV events, according to the HFA-ICOS risk tool. Cardiovascular death occurred in 4 (0.8%) patients during follow-up. CONCLUSION: This study revealed that 16-38% of patients with haematologic malignancies and breast cancer developed CTR-CVT during follow-up, in which patients with high/very high risk were well predicted by the HFA-ICOS risk assessment tool. Monitoring and managing CV risk factors are essential for safe cancer therapy.