Due to more efficient chemotherapy protocols, the number of second and even third primary carcinomas is steadily increasing. To denominate the possible origin of a carcinoma, different markers are available as an aid, e.g. hormones, proteins and lipoproteins, secretion products and cytoskeletal proteins. Cytokeratins (CKs) have gained new popularity; however, they have not been extensively evaluated in lung tumours. In our study we evaluated the staining patterns of CK polypeptides 4-8, 10, 13, 14, and 17-20 and high molecular weight (HMW) CK polypeptides in routinely processed primary lung carcinomas and lung metastases of diverse origin. As expected, immunohistochemical investigation gave no clear-cut results, but, with statistical analysis, lung adenocarcinomas could be separated from metastatic adenocarcinomas using CK 5 and 18 and HMW CK (specificity 92.5%, sensitivity 62.5%). The different origin of the metastases could often be detected using CK 18 and CK 20. Lung clear cell carcinomas and large cell carcinomas with clear cell areas could be distinguished from metastatic renal clear cell carcinomas by the CK 7 staining reaction. Squamous cell carcinomas of the lung and metastatic squamous cell carcinomas of the larynx, pharynx and oesophagus could not reliably be separated in part due to the few number of cases available. CK polypeptide typing is thus an additional aid in the differential diagnosis of lung carcinomas versus carcinomas metastatic to the lung.