The standardisation process consisted of determining tumour marker levels in all relevant population groups which are connected with the biology of the marker in normal and tumourous cells. This makes clinical application possible. ProGRP serum levels were measured in 273 healthy subjects, 176 patients with benign diseases and tumours, 200 with small cell lung cancer (SCLC), 294 with non-small cell lung cancer (NSCLC), 21 with carcinoid tumour, 93 with undifferentiated lung cancer, 35 with mixed SCLC-NSCLC, and 189 with other malignancies. ProGRP levels in patients with SCLC and SCLC-NSCLC were significantly higher than in all the other groups (p = 5.4 x 10(-3)). Moreover, in SCLC patients ProGRP levels significantly correlate with the extent of the disease and the patients' smoking habit. The cut-off level of ProGRP for SCLC is 65.89 pg/mL in the Croatian population. It is based on 96.8% specificity in benign diseases which cause problems in differential diagnosis. The sensitivity of ProGRP was 85% at the time of SCLC diagnosis.