Abstract. It has been suggested that leucocytes play an important role in the pathogenesis of complicated pancreatitis. Indeed, increased plasma concentrations of neutrophil elastase as a marker of neutrophil activation could be detected in patients with a severe course of the disease. Recently, interleukin-8 (IL-8) has been described as a novel neutrophil activating peptide. To determine the role of IL-8 in acute pancreatitis we measured its serum concentrations by a specific enzyme-linked immunosorbent assay in 10 patients with acute pancreatitis daily during the first week of hospitalization. IL-8 levels were compared with plasma concentrations of neutrophil elastase and the clinical course of the disease. Three of the patients had uncomplicated pancreatitis, while seven showed various extrapancreatic complications. Patients with complicated pancreatitis had statistically significant (P<0.05) higher mean values of IL-8 (121 ±41 pg ml-1 vs. 13 ± 6 pg ml-1, mean ± SEM) and neutrophil elastase (547 ± 35 ng ml-1 vs. 250±20 ng ml-1) than patients with uncomplicated disease. There was a positive correlation (r= 0.52, P < 0.0001) between IL-8 and neutrophil elastase in the lower concentration range of IL-8 (< 100 pg ml-1). At IL-8 levels > 100 pg ml-1 neutrophil elastase was always greatly elevated; however, under these conditions the relationship between IL-8 and elastase was no longer linear. No measurable IL-8 concentrations were found when plasma elastase was < 200 ng ml-1. During follow-up, initially elevated IL-8 concentrations decreased in correlation with clinical improvement. In conclusion, the results suggest that IL-8 contributes to initial neutrophil activation during acute pancreatitis. IL-8 seems thus to be a factor involved in the pathogenesis of complicated pancreatitis.