In female patients with idiopathic oedema the capillary filtration rate and capillary diffusion capacity for I-131 were studied in lower-limb muscles and subcutaneous tissue. The capillary filtration rate averaged 0.276 ml/100 ml per minute, and the capillary diffusion capacity for I-131 averaged 9.8 mol/(mol/ml) per 100 g per minute in the gastroonemius muscle and 8.7 mol/(mol/ml) per 100 g per minute in the lower-limb subcutaneous tissue. All of these figures are statistically significantly higher than the corresponding control findings. The results are compatible with the concept of disturbed capillary permeability, and etiopathogenetic factor of idiopathic oedema.