The detection rate of sentinel lymph nodes using isosulfan blue dye is too low for clinical use. Although exposure to radioactivity is reportedly minimal, special procedures are nonetheless required when a radioactive isotope is used as a tracer. Therefore, to eliminate the need for a radioactive tracer and to obtain a better detection rate than is currently obtained with isosulfan blue dye, we have developed a novel method that employs magnetite as the tracer. The tracer employed is ferumoxides, a colloidal superparamagnetic iron oxide of non-stoichiometric magnetite. The magnetic force within the lymph nodes was measured using a highly sensitive hand-held magnetometer ex vivo. The rate of detection of sentinel lymph nodes was 82% in non-small cell lung cancer. Intraoperative sentinel lymph node mapping using ferumoxides and a highly sensitive magnetometer is a safe, accurate, and sensitive way to detect sentinel lymph nodes. To detect sentinel lymph nodes in vivo, the magnetometer should be sterilized and the sensitivity of the magnetometer should be higher than that we used. Therefore now we are developing a more sensitive and sterilizable magnetometer.