In this study we examine the use of the concentration of thymidine kinase 1 in serum (STK1) as a prognostic factor in routine clinical settings. For this purpose we used sera from patients with oesophageal (n=101) and cardial (n=39) carcinomas and nonsmall-cell lung carcinoma (NSCLC) (n=157). Sera from healthy individuals (n=95) were used as controls. STK1 was analysed by a chemiluminiscence dot blot assay. The mean STK1 concentrations and the STK1 positive rates of the patients with oesophageal and cardial carcinomas and with NSCLC were significantly higher as compared with healthy controls (P=0.01). The mean STK1 value of oesophageal carcinoma patients correlated with T-values (P=0.021) and with stage (P<0.005), but not with grade. The mean STK1 value of cardial carcinoma patients did not correlate with grade. No data on stage and T-values were available for these patients, due to advanced disease. The mean STK1 value of NSCLC patients with squamous cell carcinoma was significantly higher as compared with adenocarcinoma type (P=0.024). The mean STK1 value of the NSCLC patients correlated with clinical grade (P=0.006), T-values (P=0.001), stage (P=0.035) and to size of the tumour (P=0.030). The mean STK1 value and the number of STK1 positive patients were also higher in recurrent NSCLC patients. There was a tendency that stage I-II NSCLC patients with an STK1 level above 2 pmol/l showed a higher frequency of recurrence/death than patients below 1 pmol/l. Our results show that STK1 is a useful marker for prognosis in patients with oesophageal, cardial and lung carcinomas.