Quantification of tumor necrosis factor-alpha (TNF-alpha) mRNA in peripheral blood mononuclear cells (PBMC) could provide information about disease activity in multiple sclerosis (MS); however, specific competitive methods must be utilized. A competitor cDNA, having the same sequence of the target TNF-alpha cDNA, a part from an internal 49-bp deletion, was generated and used to set-up a quantitative polymerase chain reaction (PCR) to quantify mRNA of TNF-alpha. Competitor and target were co-amplified using the same primers. The rates of generation of competitor and target TNF-alpha conformed closely to the prediction of the mathematical model, and a high level of accuracy and reproducibility was achieved. The method was applied to quantify TNF-alpha mRNA in PBMC of normal subjects and multiple sclerosis (MS) patients both during clinical relapses and remissions. A statistically significant higher level of TNF-alpha mRNA was detected during relapses than during remissions. High levels of TNF-alpha mRNA were found in 44% of relapses and 12% of samples during remissions, suggesting that TNF-alpha mRNA synthesis is abnormal in MS.