We have investigated the influence of fasting and lactovegetarian diet on intestinal and non-intestinal permeability in 5 patients with rheumatoid arthritis. We used low-molecular weight polyethyleneglycols (PEG 400) as probe molecules and a deterministic mathematical model to assess the permeability characteristics. Both intestinal and non-intestinal permeability decreased after fasting, but increased again during a subsequent lactovegetarian diet regime. Concomitantly it appeared that disease activity, as shown by a clinical six-joint score, first decreased and then increased again. The results indicate that, unlike lactovegetarian diet, fasting may ameliorate the disease activity and reduce both the intestinal and the non-intestinal permeability in rheumatoid arthritis.