Apoptosis results in cell death within 10 min after initiation by Bcl-2 family proteins and mitochondria; however, cells enter the apoptotic pathway at different elapsed times after being triggered. Intrinsic factors related to chemical or physical cell damage can initiate apoptosis at a specific cell cycle phase; it is not clear whether cells insulted via an extrinsic pathway also die at a specific cell cycle phase, or how apoptosis is related to cell cycle progression in cells. To illustrate the kinetic changes of apoptosis during cell cycle progression, we examined both intrinsically and extrinsically induced apoptosis in MOLT-4 and Jurkat lymphocytic leukemia cells and in cultured peripheral blood lymphocytes (PBLs) using a recently modified annexin V and propidium iodide method, which detects cell cycle-specific apoptosis. Apoptosis predominantly occurred at a specific cell cycle phase. Leukemia cells were sensitive to induction by both intrinsic (X-rays, UV light, camptothecin, arsenic trioxide, and the traditional Chinese medicine Jinke, which is an extract of Auricularia auricula) and extrinsic factors (via Fas and TNF receptor pathways). The phase at which leukemia cells entered apoptosis depended on the nature of the insult (X-ray or UV, G1-phase; camptothecin, S-phase; arsenic, G1/S phases; Jinke, G1/S phases; and TNF or Fas ligand, G1/S phases), whereas PBLs did not exhibit such insult-dependent differences. PHA-stimulated PBLs entered apoptosis, and additional cells were recruited following additional insults. Unstimulated PBLs remained unresponsive to apoptosis, and proliferating cells became insensitive to the insults after the cell cycle checkpoint was abolished by caffeine. Confluent or starving PBLs were also unresponsive to apoptotic triggers. Thus, apoptotic cell death is a cell cycle event with most, if not all, apoptosis being initiated during a particular cell cycle phase, and changes in the cell cycle result in changes in the apoptotic pattern and schedule. The coordination of apoptosis and proliferation in cells offers a mechanism for the integration of both cell cycle and apoptotic signals.