Patients with operable breast cancer were tested postoperatively to assess lymphoproliferative responses. Patients with lymphocytes capable of recognizing and proliferating in response to autologous tumor extracts had a significantly longer disease-free interval than those with a low response to their autologous extract. In this group of patients, immunological responses to autologous tumors predicted subsequent clinical course better than pathological evaluation based on tumor presence in axillary lymph nodes. Low or negative reactivity to autologous tumor often was not accompanied by a general, impaired ability to respond in LP assays. In fact, patients with low autologous tumor response and normal MLC were at particularly high risk for recurrence.