To examine the importance of immunocytochemically detectable occult axillary lymph node metastases in patients with lobular carcinoma of breast, tumor registry data from 54 cases indexed as lobular carcinoma during the period 1973-82 were reviewed. Recurrences and/or deaths due to cancer were essentially confined to the group of patients with a component of invasive lobular carcinoma (ILC), therefore this subset was selected for further study. Seven of 20 cases had lymph node metastases diagnosed histologically at the time of mastectomy. Follow-up of these patients showed four dead of disease (DOD) at one, three, three, and seven years; one alive with disease (AWD) at one year; and two with no evidence of disease (NED) at four and five years. Eleven of 20 were node negative. Follow-up of this group showed nine NED and two DOD at two and four years. Two of 20 had unknown node status. Formalin-fixed, paraffin embedded lymph node blocks were available in 12 of 20 cases with a component of ILC. Of these, 4/12 cases had histologically positive nodes while 8/12 were originally diagnosed as negative. A cytokeratin monoclonal antibody cocktail (MAK-6, CAM 5.2 and AE1/AE3) was applied to all 12 cases. Cytokeratin immunoreactivity (CK-IR) was found in all four cases that were histologically positive. Five of eight histologically negative nodes lacked CK-IR, however the other three cases showed CK-IR in micrometastases. Review of newly prepared hematoxylin-eosin sections from the paraffin blocks failed to demonstrate metastases.(ABSTRACT TRUNCATED AT 250 WORDS)