The lymph node status at mastectomy of 623 women with breast cancer was analyzed by tumor size, estrogen receptor (ER) status, host obesity, age, and smoking habits. Of the 485 women undergoing surgery who were older than 50 years of age, stepwise logistical regression analysis demonstrated that four or more positive nodes were more frequently associated with larger primary tumors (P less than 0.0001), obesity (P less than 0.001), and smoking (P = 0.0134), but not with tumor ER status (P = 0.6718). The 138 women undergoing mastectomy who were younger than 50 years of age more frequently demonstrated four or more positive nodes than older women (34.1% versus 22.9%) (P less than 0.001), after control for other factors. Among these younger women, four or more positive nodes were more frequently associated with larger tumor size (P = 0.0093), but not with obesity, smoking, or ER status. These observations suggest that tobacco usage and obesity may potentiate the early spread of malignant disease.