We have examined the occurrence of apoptotic cell death in formalin-fixed, paraffin-embedded human gastric carcinoma specimens by the terminal deoxynucleotidyl transferase (TdT)-mediated dUTP-biotin nick end labeling (TUNEL) method. The specificity of the TUNEL signals was confirmed by the omission of either TdT or biotinylated dUTP as negative controls, and by pretreatment with DNase I as a positive control. Careful observation of routine hematoxylin and eosin-stained sections showed a few tumor cells with apoptosis, especially in well-differentiated carcinomas. Intense TUNEL signals were frequently observed even in ordinary, non-pyknotic nuclei of tumor cells, and occasionally also in nuclear fragments corresponding to apoptotic bodies. Apoptotic indices (number of apoptotic cells/total number of tumor cells) ranged between 7.7 and 14.5% (mean, 10.9%) in nine well-differentiated carcinomas and between 2.7 and 7.5% (mean, 4.0%) in five which were poorly differentiated, the mean number being significantly higher in the former (P < 0.01). No apparent correlation was found between apoptosis and the expression of proliferating cell nuclear antigen, P53 or Le(y) in the present study. This high frequency of apoptosis, implying cell loss, may be related to the slow-growing nature of well-differentiated carcinomas. Poorly differentiated carcinomas, including scirrhous gastric carcinomas, showed a lower incidence of apoptosis, indicating the existence of an escape mechanism from the process.