We developed a new and automated assay for the detection of lung cancer associated cytokeratin 19 fragments in patients' sera/plasma. This new tumour marker assay CYFRA 21-1 was evaluated in technical and clinical studies using the multibatch analysers ES 300 and ES 600 from Boehringer Mannheim GmbH. The analytical performance was shown to be excellent. The clinical data from 2,037 patients demonstrate that for non-small-cell lung carcinoma CYFRA 21-1 has a higher diagnostic sensitivity compared to the established markers. Mainly for squamous cell carcinoma CYFRA 21-1 was superior (60%) to CEA (18%) or SCC (31%).