Genetic studies have previously demonstrated cytogenetic deletions and allelic imbalance or loss of heterozygosity (LOH) on the p arm of chromosome 9, in a number of tumour types. We have analysed 45 Non-Small Cell Lung Cancers (NSCLC) with a panel of highly polymorphic microsatellite markers on chromosome 9. Our results indicate that loss on 9p is concentrated within the D9S156-D9S161 region with 44% (20/45) LOH, however the area with minimal loss in this set of lung tumours was found at D9S157 (9p23), with 30% LOH (10/33), whereas loss at the IFNA locus was only found in 6% (2/34) tumours. Five of the lung tumours in this study which demonstrated LOH at D9S157 retained heterozygosity at the adjacent informative markers lying centromeric and telomeric to D9S157. No correlations were found between any of the clinico-pathological parameters and LOH on 9p or at the D9S157 locus. The results of this study indicates the presence of a further putative tumour suppressor gene on 9p at the D9S157 locus (9p23) to be most likely involved in the pathogenesis of non-small cell lung cancer.