We describe a new human tumor xenotransplant animal model that is highly efficient for engraftment, does not need host conditioning and is suitable for in vivo studies of human tumors. Pieces of 61 freshly operated primary breast tumors were implanted into 172 irradiated and 228 nonconditioned NOD/Scid mice. A high mortality was observed in irradiated but not in nonconditioned recipients. More than 90% of analyzed implanted breast cancer specimens engrafted in the NOD/Scid mice irrespective of pretreatment. The tumors were vascularized within 3 days of implantation and maintained original histomorphology as well as expression patterns of tumor markers (cytokeratin and MUC1) and cytokines (tumor necrosis factor alpha (TNF‐α), interleukin‐4 (IL‐4) and IL‐10) released by adjacent stromal cells. A majority of tumors grew slowly, locally infiltrating host tissue, whereas some grew aggressively, developing large, fatal tumor masses and metastases within regional lymph nodes. Tumor progression in mice correlated with stage, grade, proliferation index and hormone receptor status of primary tumors. The reproducible growth behavior and preservation of characteristic features suggest that this new xenotransplant model is relevant and can be recommended for testing new anticancer therapies. © 2003 Wiley‐Liss, Inc.