Objective: Triple-negative breast cancer (TNBC) is recognized to have a very poor prognosis due to its strong tendency to develop distant metastases. The present study aims to evaluate the applicability of the Nottingham Pronostic Index (NPI) in women affected or not by invasive TNBC in our centre. Materials and Methods: A retrospective study on women operated in the authors’ Department for invasive breast cancer between January 2002 and January 2007 was carried out, with a follow up of at least five years. Patients were divided into three groups based on the three prognostic categories of NPI and every category was again subdivided in TNBC or non-TNBC. Data was collected about tumor histology and biological assessment, patients characteristics, treatment, and follow up in terms of overall survival (OS). Results: The authors included 917 women operated for invasive breast cancer during this period, at a mean age of 61.4 ± 12.58 years. TNBC prevalence was 8% (71/917). Among the NPI classes, they found respectively a five-and eight-years’ OS of 99% (98-100%) and 97% (95-98%) in group 1, 88% (83-93%) and 83% (77-89%) in group 2, and 69% (59-80%) and 55% (44-70%) in group 3 (p < 0.05). After stratification of every NPI group between TNBC and non-TNBC, they found a five-years’ OS in TNBC and non-TNBC strata of respectively 94% (87-100%) and 99% (98-100%) in group 1 (p<0.05), 85% (71-100%) and 89% (84-94%) in group 2 (p0.834), and 47% (26-85%) and 73% (63-85%) in group 3 (p < 0.05). Conclusions: NPI resulted as a very simple and valuable prognostic tool and TNBC, when associated with NPI groups, significantly modified the prognostic value of NPI itself.