Etanercept (Enbrel) induces a rapid and sustained decline in disease activity in the majority of patients with refractory rheumatoid arthritis (RA). In these patients neutralization of TNFalpha and lymphotoxin (LT), previously termed TNFbeta is mediated by etanercept itself, as well as by naturally occurring soluble TNF receptors. However, the clinical response to treatment with etanercept may vary. Previously, pharmacokinetic studies have focused on the molar concentrations of etanercept, but very little is known about the kinetics of bioactive etanercept in patients treated with etanercept. The purpose of this study was to evaluate kinetics, including inter- and intraindividual variations of the total TNF binding capacity, in RA patients who were on a standard treatment schedule with etanercept.