The proliferation marker Ki-67 is one of the most controversially discussed parameters for treatment decisions in breast cancer patients. The purpose of this study was to evaluate the routine use and value of Ki-67 as a prognostic marker, and to analyze the associations between Ki-67 and common histopathological parameters in the routine clinical setting. Data from the clinical cancer registry Regensburg (Bavaria, Germany) were analyzed. Within the total data pool of 4,692 female patients, who had been diagnosed between 2005 and 2011, in 3,658 cases Ki-67 was routinely determined. Thus, a total of 3,658 patients with invasive breast cancer were included in the present study and used for statistical analysis. Ki-67 expression was associated with the common histopathological parameters. The strongest correlation was found between grading and Ki-67 ( P < 0.001). In terms of survival analyses, Ki-67 was categorized into five categories (reference category Ki-67 ≤15 %) due to a nonlinear relationship to overall survival (OS). In multivariable analysis, Ki-67 was an independent prognostic parameter both for disease-free survival (DFS) (Ki-67 > 45 %, HR = 1.96, P = 0.001) as well as for OS (Ki-67: 26–35 %, HR = 1.71, P = 0.017; Ki-67: 36–45 %, HR = 2.05, P = 0.011; Ki-67 > 45 %, HR = 2.06, P = 0.002) independent of common clinical and histopathological factors. The 5-year DFS (OS) rate was 86.7 % (89.3 %) in patients with a Ki-67 value ≤15 % compared to 75.8 % (82.8 %) in patients with a Ki-67 value >45 %. Based on the data from a large cohort of a clinical cancer registry, it was demonstrated that Ki-67 is frequently determined in routine clinical work. Ki-67 expression is associated with common histopathological parameters, but is an additional independent prognostic parameter for DFS and OS in breast cancer patients. Future work should focus on standardization of Ki-67 assessment and specification of its role in treatment decisions.