Transduction of a p53-negative H1299 human non-small cell lung cancer cell line with an adenoviral vector containing wild-type p53 (Ad5p53) induced apoptosis. Analysis of the Ad5p53-infected H1299 cells showed high levels of telomeric association prior to apoptotic nuclear fragmentation. Similar telomeric association was observed in stably transfected clones of the wtH226b cell line, which expressed exogenous wild-type p53 protein and also showed complex chromosomal abnormalities including dicentrics, rings and fragments. Fluorescence in situ hybridization (FISH) analysis using a human telomeric DNA probe indicated reductions in telomere signals in both Ad5p53-infected H1299 cells and wtH226b-S cells. In contrast, stably transfected wtH226b-AS clones expressing antisense p53 cDNA showed no telomeric association and had high levels of telomeric signals associated with a faster growing phenotype. These results suggest that wild-type p53 is involved in shortening telomeres, a possibly early event in the p53-mediated apoptotic process and in the subsequent telomeric association that predisposes a cell to genetic instability and DNA fragmentation resulting in apoptotic cell death. Moreover, loss of telomeric signals may indicate a cell's decision to undergo programmed cell death and, if so, could, serve as a sensitive marker of p53-mediated apoptosis.