Fifty-two sporadic primary non-small-cell lung carcinomas (NSCLC) were examined for microsatellite instability. Six different microsatellite markers localized on chromosomes 2, 5, 8, 10, 11 and 17 were used. Genomic instability was observed in 35% (18/52) of NSCLC at single or multiple loci. The tumors were also analyzed for p53-gene mutations by PCR-SSCP analysis. Polynucleotide stretch frameshift mutations of TGFbetaRII (transforming-growth-factor-beta receptor II), IGFIIR (insuline growth-factor II receptor) and BAX genes were also analyzed. RER+ (replication-error-positive) tumors appear not to be affected by a higher rate of point mutations in coding sequences: no correlation was found between microsatellite instability and point mutations in the p53 gene, and the RER+ tumors showed no alterations in stretches of nucleotide inside TGFbetaRII, BAX or IGFIIR.