Gastrointestinal iron absorption was measured by whole body counting in 18 patients on regular peritoneal dialysis. Ten patients received regular oral iron treatment prior to the study (iron treated group), 8 patients did not receive iron treatment (non-iron treated group). Whole body retention 14 days after oral administration of 10 microCi 59Fe together with a carrier dose of 10 mg Fe2+ was used as an estimate of absorption. The erythrocyte iron incorporation, i.e. the percentage of administered 59Fe incorporated into the total erythrocyte mass, was measured. Geometric mean iron absorption in the non-iron treated groups was 7.4+/-3.3 (S.D.) % and in the iron treated group 2.8+/-2.5% (p less than 0.01). Absorption in the non-iron treated group did not differ significantly from the value in a normal control group (p greater than 0.3). Absorption in the iron treated group was distinctly lower than in the controls (p less than 0.01), due to the high iron supplementation. Several patients in the non-iron treated group had latent or overt iron deficiency, while patients in the iron treated group had satisfactory iron status. The correlation between iron absorption and erythrocyte iron incorporation was highly significant (r=0.95, p less than 0.001). Peritoneal dialysis patients on the whole have a normally functioning iron absorption. However, due to increased iron losses and insufficient dietary iron intake, the maintenance of a satisfactory iron balance implies an adequate oral iron supplementation.