Aim: Breast carcinoma (BCA) and diabetes mellitus (DM) are two major health problems in women and the general population. Cullin-1 is reported to be an important tumor-related protein involved in cell-cycle progression, signal transduction and transcription. The aim of this work is to investigate the role of Cullin-1 in the development of BCA and to find potential relationships between Cullin-1 and diabetes in BCA patients. Methods: To evaluate the function of Cullin-1, we entered 168 patients with primary invasive BCA in this study. Pairs of BCA tissues and adjacent noncancerous tissues from these patients were collected between 2006 and 2008. We used immunohistochemistry to analyze the correlation between Cullin-1 expression and clinicopathological variables and patient survival. In addition, we investigated the role of Cullin-1 in BCA cell proliferation. Results: Cullin-1 expression was upregulated in BCA tissues. Enhanced immunoreactivity for Cullin-1 in BCA tissues was inversely correlated with overall survival and disease-free survival, which suggested a poor prognosis in BCA patients. Strong expression of Cullin-1 was more frequently observed in patients with estrogen receptor negativity and HER2 positivity. We also found that Cullin-1 expression was increased in BCA patients with a previous diagnosis of diabetes. Conclusions: Our results demonstrate that increased Cullin-1 expression is significantly correlated with poor prognosis in patients with BCA. Cullin-1 might regulate BCA cell proliferation through the ubiquitin-proteasome system. Thus, Cullin-1 might be an important marker and a therapeutic target in BCA.