We describe the case of a female patient who presented with the concurrence of two tumors, breast cancer and low grade lymphoma, in different nodes but in the same axillary lymphatic site. The sentinel node biopsy procedure for the detection of carcinoma cells in two well-identified sentinel nodes which had been colonised by lymphoplasmocytic cells was negative. Tumor collision lead to a mistake in the appropriate staging of the breast tumor by sentinel lymph-node biopsy. We believe that sentinel lymph-node biopsy should be avoided in cases of lymphoma.