The major prognostic indicator in patients with breast cancer is the presence of metastases in axillary lymph nodes. The authors developed a mathematical model, based on 1446 complete axillary dissections performed in Milan between 1983 and 1986, and determined the following: (1) the sample size from Level I necessary for a 90% certainty degree of N0 axillary status; (2) the probability of residual tumor in the axilla after axillary sampling from Level I; and (3) the maximum number of involved axillary nodes in Levels I, II, and III to be expected (90% certainty) after sampling from Level I. Thus, this model permitted the determination of the cutoff level for a true N0 axillary status when only a few nodes are sampled from Level I. The cutoff level for a T1 primary tumor is ten axillary nodes removed and found uninvolved. Also, this model provides guidance in managing possible residual tumor after an incomplete axillary dissection. This information is important in indicating adjuvant axillary radiation therapy and chemotherapy or hormone therapy.