203 primary human lung tumours, of which 119 were adenocarcinoma and 84 were squamous cell carcinoma, were investigated immunohistochemically for the expression of c-erbB-2 protein. Positive staining was evident in 33 (28%) of adenocarcinomas and 2 (2%) of squamous cell carcinomas. In cases of adenocarcinoma, c-erbB-2 was present in 18% of those with stage I disease. In stage IIIA, stage IIIB and stage IV cases, c-erbB-2 was present in 39%, 50% and 60%, respectively (I vs. IIIA and I vs. IIIB: P less than 0.05, I vs. IV: P less than 0.01). The 5-year survival rates of c-erbB-2 positive patients and those who were negative were 30% and 52%, respectively, with a statistically significant difference (P less than 0.01). These observations suggest that when the expression of c-erbB-2 correlates with invasiveness of the tumour, this correlation may serve as a prognostic indicator, particularly in cases of adenocarcinoma of the lung.