Serum concentrations of soluble ICAM-1 were measured by an enzyme-linked immunosorbent assay in 80 patients with non-small cell lung cancer (NSCLC). The mean serum concentration of soluble ICAM-1 in 80 patients with NSCLC was 472.8 +/- 370.8 ng ml-1 (range 75.6-3177.4 ng ml-1), whereas it was 196.8 +/- 54.6 ng ml-1 (range 128.1-276.4 ng ml-1) in 10 healthy controls. Sixty of 80 patients with NSCLC (75.0%) showed elevated concentrations (more than 306 ng ml-1, mean+2 SD in controls). The differences in mean serum concentration and positive rate between control subjects and NSCLC patients were significant (P = 0.0001). Serum ICAM-1 concentrations showed a significantly positive correlation with primary tumour size (maximum diameter) (P = 0.0209). Although the difference was not significant, the overall survival of patients with low serum ICAM-1 concentrations (< 306 ng ml-1) tended to be longer than that of patients with high concentrations (> or = 306 ng ml-1). These results suggest that serum ICAM-1 may be useful for serological diagnosis, monitoring of tumour volume or quantity in patients with NSCLC.