Bortezomib is a novel anti-cancer agent which has shown promising activity in non-small lung cancer (NSCLC) patients. However, only a subset of patients respond to this treatment. We show that NSCLC cell lines are differentially sensitive to bortezomib, IC50 values ranging from 5 to 83 nM. The apoptosis-inducing potential of bortezomib in NSCLC cells was found to be dependent not only on the apoptotic phenotype but also on the proteasomal phenotype of individual cell lines. Upon effective proteasome inhibition, H460 cells were more susceptible to apoptosis induction by bortezomib than SW1573 cells, indicating a different apoptotic phenotype. However, exposure to a low dose of bortezomib did only result in SW1573 cells, and not in H460 cells, in inhibition of proteasome activity and subsequent apoptosis. This suggests a different proteasomal phenotype as well. Additionally, overexpression of anti-apoptotic protein Bcl-2 in H460 cells did not affect the proteasomal phenotype of H460 cells but did result in decreased bortezomib-induced apoptosis. In conclusion, successful proteasome-inhibitor based treatment strategies in NSCLC face the challenge of having to overcome apoptosis resistance as well as proteasomal resistance of individual lung cancer cells. Further studies in NSCLC are warranted to elucidate underlying mechanisms.