Intestinal permeability was assessed before and 1, 2, 4, 8 and 12 weeks after commencing a gluten-free diet (GFD) in eight coeliac subjects. Intestinal morphology was quantified in six coeliac subjects on a normal diet, six coeliac subjects on a GFD, and 21 normal subjects. T-cell activity was measured in the eight coeliac subjects by soluble interleukin-2 receptor (sIL-2R) concentration (normal less than 477 U/mL). Intestinal permeability was increased 10-fold with a geometric mean value of 0.72 on a normal diet, and decreased to 0.17 at 4 weeks (P = 0.04), to 0.07 at 8 weeks (P = 0.010), and to 0.20 at 12 weeks (P = 0.015) of a GFD. Two of the eight subjects showed a poor response to gluten withdrawal. Quantitative intestinal morphology showed no significant improvement after 3 to 6 months of a GFD. Mean +/- s.d. sIL-2R concentrations in the eight subjects were increased 5-fold higher than control values at 1400 +/- 530 U/mL on a normal diet and decreased to 750 +/- 200 U/mL after 12 weeks of a GFD (P = 0.004). We conclude that intestinal permeability improves rapidly in the majority of coeliac subjects after commencing a GFD, although some abnormal permeability and increased T-cell activity persists. This may be due to varying degrees of gluten ingestion resulting in continued immune activation.