Recently, a new immunometric assay (Cyfra 21-1) was developed to measure serum concentrations of a soluble fragment of cytokeratin subunit 19. With this method, supplied by Boehringer Mannheim (EIA Test Cyfra 21-1), an Italian multicenter trial was performed in patients with lung cancer. Cyfra 21-1 serum levels were determined in 568 normal subjects (blood donors), 607 patients with non-malignant diseases (491 respiratory diseases) and 730 patients with malignancies. In the latter group 584 had lung cancer. All these 584 patients had pathologically confirmed disease; 314 were epidermoid tumors, 166 adenocarcinomas, 88 small cell cancers and 16 large cell cancers. In the 568 healthy blood donors the mean Cyfra 21-1 value was 0.91 ng/ml (SD 0.47 ng/ml; range 0.05-2.90 ng/ml). A threshold of 1.9 ng/ml was chosen as the upper limit of normality. High levels of Cyfra 21-1 were observed in patients with chronic hepatitis (positivity rate: 17/51-33.3%) and with pancreatitis (positivity rate 5/16-31.3%). In 114 out of 491 (23.2%) patients with respiratory diseases Cyfra 21-1 showed values greater than 1.9 ng/ml. The overall sensitivity (all stages) of Cyfra 21-1 in lung cancer was 65.6% (383/584). When the histology was considered the highest positivity rates were found in patients with squamous cell tumors (226/314; 72%) followed by adenocarcinomas (105/166; 63%). In patients with SCLC the global sensitivity was 52.3% (46/88). Higher sensitivity of Cyfra 21-1 was observed from stage I to stage IV (53.9% vs 85.7%; Chi square: p < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)