Many laboratories have demonstrated that polypeptide growth factors stimulate human cancer cell growth in experimental systems. Despite this observation, a central question remains: can inhibition of peptide growth factor action inhibit tumor growth in humans? To answer this question, several other concerns must first be addressed. Which growth factors are critical for tumor growth? What are the specific cellular effectors for each growth factor? Can feasible therapies be designed to interrupt growth factor pathways? This issue of Breast Cancer Research and Treatment explores the relevance of the insulin-like growth factors to breast cancer cell growth.