The PI3K/AKT signaling pathway is one of the most deregulated pathways in cancer. Nearly 30% of breast cancers show a genetic alteration in the PIK3CA gene. Hotspots of mutations were described in exon 9 and exon 20. Prevalence and prognostic impact of the PIK3CA mutations are controversially discussed. In this study we describe the prevalence of PIK3CA mutations in a cohort of breast cancer patients and its association to tumor characteristics and prognostic factors. 806 tumor samples were obtained from a cohort of 795 breast cancer patients. Tumor DNA was analyzed by PCR. Patient data, tumor characteristics and prognostic factors were obtained from patients charts. We found a mutation rate of 25% in the entire cohort. Tumors with a PIK3CA mutation were significantly associated with HR positivity (p=0,001), HER2 negativity (p=0,020) and low grading (p=0,001). No significant association was found to age, menopausal status, tumor stage und nodal status. In this study tumors harbouring a mutation of the PIK3CA-gene, were associated to prognostic more favourable factors.