Lymph nodes in breast cancer have been examined by two histological techniques in 324 patients who underwent axillary node dissection. A routine pathologic examination consisted in the examination of only one pathological section while in the other, the lymph nodes were macroscopically serially sectioned. With the latter, an increase of 18.8 per cent in the number of detected metastases was noted. However this increase can be considered of prognostic and therapeutic importance only in 8.6 per cent of cases: half corresponding to those cases in which more than the initial threshold number of three lymph nodes were found, the other half corresponding to those cases in which one lymph node was found while the initial examination was negative. In the latter case, the increase of lymph node involvement was mainly due to "clandestine" metastases, i.e. lymphatic embolisation of micrometastases.