Patients with cancer frequently show glucose intolerance. This study is to estimate the status of total diabetes and prediabetes in breast cancer patients after systemic treatment through an oral glucose tolerance test (OGTT) in China. All the 119 breast cancer patients more than 3 months after systemic treatment with surgery and chemotherapy participated in the study. All the patients without the diagnosis of diabetes underwent OGTT, and fasting and 2-h glucose levels were measured to identify undiagnosed diabetes and prediabetes. Previously diagnosed diabetes were determined on the self-report and the medical record. Of the 119 breast cancer patients, with the median age of 50.1 years and the mean age of about 48 years when they were initially diagnosed with breast cancer, which showed the similar characters of China and Asia breast cancer patients, the overall incidences of total diabetes and prediabetes were 21.8 and 43.7 %, respectively. About 80 % of the diabetes were previously undiagnosed. About 80.0 % of the cases of undiagnosed diabetes and prediabetes met the criteria for elevated 2-h plasma glucose levels through OGTT but not the criteria for elevated fasting glucose levels. Our study firstly documents high incidences of previously undiagnosed diabetes and prediabetes in breast cancer patients during follow-up after systemic treatment through OGTT, indicating that greater diabetes screening, especially through OGTT, prevention, and treatment strategies among breast cancer patients, after systemic treatment for these patients is needed.