To evaluate the possible use of mutant ras as a biomarker for lung cancer, we have analyzed “normal appearing” lung tissue, lung tumor, lung metastases and sputum samples from patients with non‐small cell lung cancer (NSCLC). As a control, we used lung tissue and sputum samples from patients without oncological diseases or lung disorders. Our analyses were performed with the aid of enriched PCR (EPCR), a method which enables detection of ras mutation even if present at low incidence. EPCR identified K‐ras codon 12 mutations in 10% of lung tissues obtained from patients with no lung diseases, whereas the same mutation was detected in 60% of samples of normal appearing lung tissues obtained from patients with NSCLC, 62% of NSCLC tumors and 80% of metastases. Analysis of sputum samples of patients with NSCLC identified 47% to harbor mutant ras allele, whereas 12.5% of controls diagnosed with non‐oncological lung diseases carried this mutation. Most of these mutations were detected with the aid of EPCR only, indicating that a minority of cells in a given sample harbor this mutation. The ability to detect K‐ras codon 12 mutation in 60% of lung tissue samples and in 47% of sputum samples taken from patients with lung cancer (as compared with 10% and 12.5% of respective controls) points to the potential use of ras mutation as a biomarker for exposure and possible identification of patients who may be at a higher risk of developing lung cancer. © 1995 Wiley‐Liss, Inc.