Interstitial fluid pressures were measured by the wick in needle method in the swollen and normal arms of 38 patients with lymphoedema resulting from treatment for breast cancer. The mean increase in arm volume, calculated from sequential circumferential measurements, was 33% (range 0.25 to 85.9). Subcutis interstitial fluid pressure in the swollen arm (2.0 cmH2O range -4.5 cmH2O to 6.8 cmH2O) was significantly greater (p < 0.001) than in the contralateral, non-swollen arm (-2.6 cmH2O range -11 cmH2O to 0 cmH2O). Interstitial fluid pressure in the oedematous arm did not correlate with the duration of the swelling (1-324 months), but did correlate with the increase in volume relative to the normal arm (r = 0.38, p < 0.05), the slope ('apparent compliance') being 28 ml.dl-1.cmH2O-1. The pressure-volume curve was less steep than the classic curve for acute oedema of dog limbs (Guyton, 1965). Vascular pressures were normal. The interstitial fluid pressures were not as high as those reported for lymphoedema of the lower extremity (mean 17.9 mmHg, Christensen et al., 1985). Nevertheless, the rise in interstitial fluid pressure by an average of 4.6 cmH2O constitutes a force opposing further microvascular fluid filtration and perhaps promoting fluid drainage out of the arm.