The effects of fasting for 7 days were investigated in 13 patients with rheumatoid arthritis (RA) in comparison with a control regimen in a cross-over trial. The effects of fasting on clinical performance and blood neutrophil functions were studied. During fasting, with a mean weight loss of 5.1 kg, clinical inflammation in the joints and the erythrocyte sedimentation rate (ESR) decreased. During the control period the joints either remained unchanged or deteriorated, and no change was observed in the body weight or the ESR. The locomotion of neutrophils under agarose, induced by a reference serum, decreased during the fasting period (p less than 0.001), but no change in their locomotion was induced by an Escherichia coli bacterial factor. During the control period, however, the locomotion induced by either stimulant was significantly decreased. Generation of migration-stimulating factors from the patients' plasma declined 3 days after the end of fasting (p less than 0.001). The adherence of the neutrophils to nylon fibres was unchanged during both periods. The bactericidal capacity improved during fasting, both in comparison with the initial value (p less than 0.005) and with the values from the control period (p less than 0.001). An association was found between improvement in inflammatory activity of the joints and enhancement of neutrophil bactericidal capacity. Fasting appears to improve the clinical status of patients with RA. This could partly be due to the observed changes in the functions of the neutrophils, since the latter contribute to the inflammatory joint reactions.