CAPD-related peritonitis was used as an in-vivo model to study Il-8 during peritoneal inflammation. Eleven episodes were studied in nine patients, who were followed on 8 consecutive days from the start of peritonitis and once after recovery (control). Il-8 was measured in dialysate (night dwells) and serum. The Il-8 time course was compared to Il-6 and TNF alpha. In addition, an in-vivo relationship between dialysate Il-8 and intraperitoneal accumulation of neutrophils was studied. A highly increased peritoneal appearance rate of Il-8 was found in the acute phase that decreased to control values during recovery. A remarkable parallelism was observed for dialysate Il-8 and Il-6 with respect to the time course and the peritoneal appearance rate. In contrast, the appearance rate of TNF alpha was much less and had a different time course. In three of four cases where the dialysate Il-8 peak occurred on day 2, the dialysate Il-6 peak still coincided with Il-8, in contrast to TNF alpha (always day 1). Dialysate Il-8 generally exceeded serum concentrations during the entire follow-up, indicating intraperitoneal Il-8 synthesis. A positive correlation was present between the dialysate Il-8 peak and the maximal number of neutrophils in dialysate. This relationship was absent for Il-6 and TNF alpha. In five of six episodes where neutrophils were quantified on both day 1 and 2, the Il-8 peak occurred simultaneously with the neutrophil peak. These findings suggest that Il-8 is involved in the recruitment of neutrophils towards the dialysate during peritonitis.