To clarify the significance of blood group antigen A (BAA) expression by neoplastic cells, we studied patients who had curative resections of stage I non-small cell lung carcinomas. Immunohistochemical staining using monoclonal antibodies was used to detect BAA expression by paraffin-embedded carcinoma cells. One hundred three patients were studied; mean age was 62.6 years, and 70 (68%) were male. Histologic types were as follows: adenocarcinoma, 52 (50.5%); squamous cell, 25 (24.3%); large cell, 24 (23.3%); and adenosquamous, 2 (1.9%). Histologic grades were as follows: I, 13 (12.6%); II, 26 (25.3%); and III, 64 (62.1%). All patients had American Joint Committee on Cancer stage I tumors: 65 patients (63.1%) had T1 tumors, and 38 (36.9%) had T2 tumors. Recurrences developed in 25 (24.3%) and metachronous malignancies in 4 (3.9%). Survival was 75% +/- 4.8% at 3 years and 66.6% +/- 7.5% at 5 years. Eighty-nine patients (86.4%) were blood group A and 14 (13.6%) were AB. Ninety-five (92.2%) were secretors of BAA and 8 (7.8%) were not. The expression of BAA by neoplastic cells was not detectable in 34 (33%), trace (1% to 5% of neoplastic cells) in 10 (9.7%), 1+ (6% to 25%) in 8 (7.8%), 2+ (26% to 50%) in 12 (11.7%), 3+ (51% to 75%) in 12 (11.7%), and 4+ (76% to 100%) in 27 (26.2%). The pattern of neoplastic cell staining was homogeneous in 14 patients (20.3%) and heterogeneous in 55 (79.7%). Carcinoma recurrence, overall survival, and event-free survival were not related to secretor status, BAA expression, or pattern of staining.(ABSTRACT TRUNCATED AT 250 WORDS)