The H69CIS200 and H69OX400 cell lines are novel models of low-level platinum drug resistance developed from H69 human small cell lung cancer cells with eight 4-day treatments of 200 ng/ml cisplatin or 400 ng/ml oxaliplatin respectively. A recovery period was given between treatments to emulate the cycles of chemotherapy given in the clinic. The resistant cell lines were approximately 2-fold resistant to cisplatin and oxaliplatin and were cross resistant to both drugs. Platinum resistance was not associated with increased cellular glutathione, decreased accumulation of platinum or increased DNA repair capacity. The H69 platinum sensitive cells entered a lengthy 3 week growth arrest in response to low-level cisplatin or oxaliplatin treatment. This is an example of the coordinated response between the cell cycle and DNA repair. In contrast the H69CIS200 and H69OX400 cells have an alteration in the cell cycle allowing them to rapidly proliferate post drug treatment. The resistant cell lines also have many chromosomal rearrangements most of which are not associated with the resistant phenotype, suggesting an increase in genomic instability in the resistant cell lines. We hypothesised that there was a lack of coordination between the cell cycle and DNA repair in the resistant cell lines allowing proliferation in the presence of DNA damage which has created an increase in genomic instability. The H69 cells and resistant cell lines have mutant p53 and consequently decrease the expression of p21 in response to platinum drug treatment, promoting progression of the cell cycle instead of increasing p21 to maintain the arrest. A decrease in ERCC1 protein expression and an increase in RAD51B foci activity was observed with the platinum induced cell cycle arrest and did not correlate with resistance or altered DNA repair capacity. These changes may in part be mediating and maintaining the cell cycle arrest in place of p21.The rapidly proliferating resistant cells have restored the levels of both these proteins to their levels in untreated cells. We use the term ‘regrowth resistance’ to describe this low-level platinum resistance where cells survive treatment through increased proliferation. Regrowth resistance may play a role in the onset of clinical resistance.