RhoA protein is over-expressed in breast cancer and other solid tumors and has been used in tumor biopsies as a quantitative tumor marker for progression, stage and prognosis in molecular detection strategies. Measuring protein markers in plasma or blood cells is preferred to tumor biopsies as it represents a minimally invasive, repeatable measurement that can be followed over time. In this study we evaluated the hypothesis that quantitative RhoA protein expression in circulatory lymphocytes is identically associated with the same tumor clinico-pathological features found in biopsies. RhoA protein levels were analyzed by Western blotting in circulating lymphocytes isolated from 52 consecutive patients with breast cancer and in 34 paired breast tumor biopsies from the same case study, and compared with the following clinico-pathological features of the patients: histological grade, tumor size, steroid receptor status, lymphonode status, proliferative activity and prognosis [Nottingham Prognostic Index (NPI)]. We observed that the level of circulatory, peripheral lymphocyte RhoA expression reflected that found in the matched biopsy of the same patient. Furthermore, similarly to previous reports regarding breast cancer tissue biopsies, the level of RhoA protein expression in both biopsies and in circulatory lymphocytes was positively associated with tumor size, grade, proliferative activity of the tumor biopsy and NPI, while there was no significant association of RhoA protein expression with either estrogen- or progesterone-receptor expression. Our study demonstrated that the association of lymphocyte RhoA protein expression with classical clinico-pathological parameters closely corresponded with that observed for RhoA protein expression in the tumor biopsies. We propose that measurement of RhoA expression in the circulatory lymphocytes of breast cancer patients can be used to predict breast cancer occurrence, progression and prognosis and may prove valuable in the management of cancer patients.