Soluble cytokeratin fragment 19 levels were measured with an enzyme immunoassay method developed by Boehringer Mannheim (Enzymun-Test CYFRA 21-1) in the serum of 185 patients with lung cancer [149 with non-small-cell lung cancer (NSCLC) and 36 with small-cell lung cancer (SCLC)] and 97 patients with benign lung diseases in order to determine its clinical usefulness in the diagnosis of lung cancer and follow-up of treatment. We used the cut-off value of 3.5 ng ml-1, established by the Japan CYFRA research group. This cut-off value is based on calculations using the receiver operating characteristic approach instead of using the 95% specificity approach recommended by other authors. The resulting sensitivity and specificity for the group of all lung cancer patients were 65.4% and 84.5% respectively. The sensitivity was highest (76.1%) for squamous cell carcinoma and lowest (44.4%) for SCLC. For NSCLC patients, when CYFRA 21-1 levels were analysed by node (N) factor, patients who presented with mediastinal lymph node metastasis (N2 or N3) demonstrated higher serum CYFRA 21-1 levels (5.6; interquartile range 3.2-11.5 ng ml-1) than patients without mediastinal node metastasis (N0 or N1, 3.9; interquartile range 2.2-10.0 ng ml-1; Mann-Whitney U-test, P = 0.0373). We compared the discriminatory power of CYFRA 21-1 with that of other tumour markers including carcinoembryonic antigen (CEA), squamous cell carcinoma antigen (SCC) and neuron-specific enolase (NSE). The area under the curve (AUC) of each ROC curve was calculated using the CLABROC program for statistical analysis. CYFRA 21-1 appeared to have the most discriminatory power of the markers tested in the diagnosis of lung cancer. In serial measurements of 14 patients receiving chemotherapy or radiotherapy, a high degree of correlation was noted between serum levels of CYFRA 21-1 and extent of clinical response (Wilcoxon, P = 0.0093).