Fifteen per cent of metastatic breast cancer will develop symptomatic leptomeningeal metastases. The introduction of trastuzumab (Herceptin) therapy has improved the response rates of survival of patients with metastatic breast cancer overexpressing HER2. Although previous studies are retrospective and of limited number, involving small study groups and different types of patient management, several authors have reported a 30% incidence of leptomeningeal metastases in patients with metastatic breast cancer overexpressing HER2 who were treated with trastuzumab, while 70 to 80% of cases of the disease were controlled systemically. In order to improve control of the disease at the level of the central nervous system (CNS), routine detection of leptomeningeal metastases in high-risk patients could be offered. CA 15-3 in cerebrospinal fluid (CSF) detection might be useful in helping to diagnose CNS metastases, particularly where cytology results are negative--which applies to 30% of cases--because tumor markers are more sensitive in detecting the tumor process. Our study validate CA 15-3 measurement in CSF and reference values were given.