Peritoneal function in relation to the occurrence of peritonitis and the osmolarity of the dialysate was studied in 72 CAPD patients with a mean duration of treatment of 16.5 +/- 9.0 months (group 1). Data from 24 of these patients, who were on CAPD for longer than 2 years (average 28.6 +/- 4.9), were subjected to further detailed analysis (group 2). Each group consisted of two subgroups, one of patients who had experienced peritonitis and one of patients who had had no episodes of peritonitis. Results from group 1 revealed that the use of hyperosmolar bags increased in parallel with the duration of CAPD treatment even in the non-peritonitis subgroup, and that peritonitis enhanced the tendency to use hyperosmolar dialysate solutions. This phenomenon was also observed in the peritonitis subgroup of group 2, but was not apparent in the non-peritonitis subgroup of group 2 when examined as a whole: however, individual analysis revealed that some of them had a similar tendency to use hyperosmolar dialysate, as was seen in the peritonitis subgroup. These results confirm that the peritonitis impairs the ultrafiltration capability of the peritoneum. The results also suggest that the long-term use of hyperosmolar dialysate may be associated with decreased ultrafiltration, hence emphasis should be placed upon the use of hyperosmolar dialysate solutions for long-term CAPD.