Little quantitative data exist on the extent of apoptosis (genetically-mediated cell deletion) in different human tumor types. Hematoxylin and eosin-stained paraffin sections of 102 malignant tumors (58 types) were evaluated for apoptotic cells and apoptotic bodies, using the 40x objective with a calibrated eye-piece and avoiding necrotic zones. The percentage of apoptotic cells and apoptotic bodies in the total number of tumor cells examined was designated as the apoptotic index (AI) for each case. There was a wide range in the AI for different tumor types: 45 tumors had AI < 1% and 93 had an AI of < 7%. In 107 additional tumors (11 types), the AI was determined to be within the same low, intermediate, or high range as the index cases. Apoptotic nuclear material was usually more prominent than mitoses. These results suggest that each tumor type has a characteristic AI that reflects innate tumor cell susceptibility to undergo apoptosis. Additional data are needed to determine whether significant variations in AI correlate with altered proliferative indices, aberrant oncogene/tumor suppressor gene expression, and standard clinicopathologic variables.