This analysis of 32 pairs of human squamous cell lung carcinomas and normal matched control DNA demonstrates that loss of heterozygosity (LOH) is infrequent at the nm23-H1 locus, affecting only 2 of the 18 informative cases. Both LOH cases were in the tumor stage IIIA. One tumor was of poor and the other of moderate histological grade. These and an additional 34 tumor samples were also analyzed immunohistochemically for the presence of nm23-H1 protein. Of the 66 cases tested for the presence of nm23-H1 protein 54 were negative. Eight samples exhibited up to 35% positive cells (with weak immunostaining intensity) and four between 35% and 70% (moderate immunostaining intensity); no sample showed more than 70% positive cells. Noncancerous lung parts contained no nm23-H1 protein. nm23-H1 expression was independent of TNM stage, grade, tumor size, and patient's survival. Two samples with LOH were negative for nm23-H1 protein. We therefore conclude that neither loss of heterozygosity of the nm23-H1 gene nor the intensity of specific protein expression are related to squamous cell lung carcinoma development and progression.