We assessed the status of P53 in 32 surgically treated non-small cell lung cancers (NSCLC) by using yeast functional assay. For functional assay, total RNA extracted from fresh-frozen specimens was reverse transcribed and P53 cDNAs were PCR-amplified using Pfu DNA polymerase (Stratagene). The transcriptional competence of the P53 cDNA was then tested in a yeast reporter strain. 20 of the 32 (69%) NSCLC patients contained mutant P53 in the yeast functional assay with the higher frequency in squamous cell carcinoma (14/17, 82%) than in adenocarcinoma (5/10, 50%) and large cell carcinoma (3/5, 60%) (p<0.01, chi2 test). No significant difference was observed with respect to the TNM. Preliminary survival analysis showed that patients scoring positive for the yeast test had shorter disease-free survival (median = 10 months) than those that scored negative (median > 21 months). Our results suggest that yeast functional assay is not only an improved method to examine the status of P53, but might hopefully improve understanding of the role of mutant P53 in the clinical evaluation of NSCLC.