Potentially lethal damage (PLD) and its repair were studied in confluent human fibroblasts by analyzing the kinetics of chromosome break rejoining and misrejoining in irradiated cells that were either held in noncycling G₀ phase or allowed to enter G₁ phase of the cell cycle immediately after 6 Gy irradiation. Virally mediated premature chromosome condensation (PCC) methods were combined with fluorescence in situ hybridization (FISH) to study chromosomal aberrations in interphase. Flow cytometry revealed that the vast majority of cells had not yet entered S phase 15 h after release from G₀. By this time some 95% of initially produced prematurely condensed chromosome breaks had rejoined, indicating that most repair processes occurred during G₁. The rejoining kinetics of prematurely condensed chromosome breaks was similar for each culture condition. However, under noncycling conditions misrepair peaked at 0.55 exchanges per cell, while under cycling conditions (G₁) it peaked at 1.1 exchanges per cell. At 12 h postirradiation, complex-type exchanges were sevenfold more abundant for cycling cells (G₁) than for noncycling cells (G₀). Since most repair in G₀/G₁ occurs via the non-homologous end-joining (NHEJ) process, increased PLD repair may result from improved cell cycle-specific rejoining fidelity of the NHEJ pathway.