The authors present a prospective study of the variations in sensitivity that appear in the armpit and arm after the intercostobrachial nerve has been sectioned in a modified radical mastectomy. Of a total of 208 patients studied between 1978 and 1987, the intercostobrachial nerve was sectioned in 139 patients, whereas in 30 it remained intact; in 39 patients only peripheral branches of the nerve were sectioned. The patients were examined at regular postoperative intervals in order to evaluate their sensitivity both to touch and to pain in the axilla and arm. Four hundred thirty-three examinations were carried out and the 3 operative groups were assessed accordingly. After the nerve was sectioned, there was increasing anesthesia in the armpit and hypoesthesia on the posterointernal face of the arm, while for the patients in whom the intercostobrachial nerve remained intact, these alterations were less intense and long-lasting. Significant statistical differences between the two groups also exist. In the absence of axillary lymphatic ganglia infiltrated by the tumor, the conservation of the intercostobrachial nerve is recommended.