PURPOSE : Recent mammographic equipment have been of good quality and yielded high diagnostic accuracy for thedetection of breast cancer. However, negative mammogram does not necessarily rule out breast cancer. Therefore wereviewed cause of false negative mammography in confirmed breast cancer to improve diagnostic accuracy and foradequate clinical approach. MATERIAL AND METHODS: We reviewed 19 cases of confirmed breast cancer, which showedfalse negative mammography with positive sonographic findings. Retrospective analysis was done by correlating thepatient's age, sonographic finding and mass size, mammographic breast pattern and cause of false negativemammogram, and clinical symtoms. RESULTS : Among the 5 patients below 35 years in age, mass was not visible due todense breast in 4 and due to small size in 1 case. In 14 patients over 35 years in age, 11 had normal mammographicfindings, 4 had dense breast, and 7 had small sized mass. Remaining 3 cases showed asymmetric density in 2 andarchitecture distortion in 1 case. All showed mass lesion in sonography ; ill defined malignant appearance in 14,well defined malignant appearance in 2, and well defined benign in 3 cases. CONCLUSION : Negative mammogram shouldbe correlated with sonography in case of dense breast, below 35 years in age with palpable mass and under risk forbreast cancer.