To evaluate the occurrence of apoptotic bodies in endocervical adenocarcinoma in situ (AIS) and investigate the relationship of apoptosis to mitotic activity, we performed counts of apoptotic bodies and mitotic figures in 43 patients with AIS and in a comparable control group with nonneoplastic endocervical glandular epithelium. The ages of the patients with AIS ranged from 27 to 74 years (mean = 40). Mitotic figures were present in all AIS cases, and apoptotic bodies were seen in all but two extremely small lesions. In 28 AIS cases in which lesions were large enough to count 10 consecutive high-power fields (HPF), counts of apoptotic bodies ranged from 1 to 36/10 HPF (mean and median = 16), and counts of mitotic figures ranged from 1 to 53 mitotic figures/10 HPF (mean and median = 18). Counts of apoptotic bodies correlated directly with counts of mitotic figures. The ages of the 28 control patients ranged from 32 to 56 years (mean = 43). Counts of apoptotic bodies in the control cases ranged from 0 to 10 per case (mean = 1.1). Apoptotic bodies were present in only 13 (46%) control cases. The highest counts in these cases ranged from 1 to 6 apoptotic bodies/10 HPF (mean = 2.3). Mitotic figures were present in only 4 (14%) control cases. The counts in these cases ranged from 1 to 3 mitotic figures/10 HPF (mean = 1.8). Counts per 10 HPF were significantly (p < 0.001) more for AIS cases than for controls for apoptotic bodies and mitotic figures. Our results indicate that apoptotic bodies, as well as mitotic figures, occur almost universally in AIS. Both occur significantly more often and in greater numbers in AIS than in nonneoplastic endocervical glandular epithelium. Apoptotic bodies are a consistent morphologic feature of AIS, and their identification may be diagnostically useful.