9 patients were observed prospectively during 14–40 months 003continuous ambulatory peritoneal dialysis (CAPD) treatment. From start of CAPD, each patient recorded dwell time, drained ultrafiltration volume (ΔV), initial glucose concentration in dialysate, daily fluid intake, body weight and blood pressure on a special form. These data, together with monthly values for albumin, urea, creatinin, phosphate, glucose and β2-microglobulin in plasma and in instilled dialysate, were later fed into a specially designed computer program to compare changes in the monthly mean ( ± SEM) values. During 5 episodes of peritonitis, daily changes in ΔV were also computed. A long-term increase in ΔV was found in 4 and a decrease in 5 patients. In all 9 patients ΔV changed intermittently. All changes were most pronounced for long dwell times as compared to shorter dwell exchanges. The decrease in ΔV started within the first 12 months of treatment. In the daily routine we were aware of decreased ultrafiltration capacity in 3 patients only. Intermittent monthly changes in ΔV could partly be correlated to changes in daily fluid intake. No correlations were found between long-term changes in ΔV and fluid intake. All except 1 patient gained progressively in body weight, but without correlations to fluid balance, blood pressure and plasma albumin concentration. At the start of the observation period, most patients loosing ΔV during this study appeared to have a more permeable membrane with a higher absorption rate of glucose and higher equilibration ratios for creatinine and β2-microglobulin in 5-hours drained dialysate as compared with the other patients. However, this was not statistically different between the two groups of patients. During the observation period, most patients with decreased ΔV also increased transperitoneal solute transport, while the solute transport decreased in patients with increasing ΔV, but these changes were only significant for some patients. During peritonitis, ΔV decreased significantly 1 day before any other signs of peritonitis. All changes in ΔV were most pronounced for long dwell times as compared with short dwell times. It is suggested that changes in ultrafiltration can be related to altered permeability of the peritoneal membrane, which appear earlier and more frequent than suggested by others, and any loss of ΔV can be explained by a more permeable (‘open’) peritoneal membrane. It is also possible that different diseases act differently on the permeability of th