Using a panel of 20 non-small cell lung cancer (NSCLC) cell lines established from previously untreated patients, we investigated the relationships between intrinsic chemoresistance (to four agents used commonly in the therapy of NSCLC) and HER-2/neu gene expression (which encodes glycoprotein p185neu), p53 gene mutations, and cell proliferation characteristics. Our results demonstrated that high p185neu expression was correlated with chemoresistance, low S-phase fractions, and long doubling times. By contrast, cell lines expressing relatively low levels of p185neu were relatively chemosensitive and had higher S-phase fractions and shorter doubling times. Although mutation of the p53 gene was a common event in this panel of cell lines (present in 18 of 20 lines), there was no relationship between mutations at any specific codon and chemoresistance or cell proliferation characteristics. Multivariate analysis revealed that the level of p185neu was the only independent predictor for chemoresistance to doxorubicin, etoposide, and probably cisplatin. Although intrinsic chemoresistance almost certainly is a multifactorial process, overexpression of p185neu may be an important factor in the chemoresistance of NSCLC.