IL-15 is a proinflammatory cytokine which has recently been implicated in multiple sclerosis (MS) pathogenesis, where it may play a role in the initiation and/or progression of the disease. We have used reverse transcriptase-polymerase chain reaction (RT-PCR) to study IL-15 mRNA levels in peripheral blood mononuclear cells (PBMC) from healthy controls and relapsing-remitting MS (RRMS) patients in a stable phase of the disease and during a bout, both before and after corticosteroid treatment (CST). IL-15 mRNA expression was found to be similar in controls and stable patients. We have detected an increased level of IL-15 mRNA in PBMC of patients with a relapse, which was maintained after CST. We have also found an inverse correlation between PBMC IL-15 mRNA levels at the onset of the relapse and the time elapsed since the previous attack, as well as an absence of correlation between IL-15 mRNA levels and the patient demographic and clinical characteristics. Results in the present work further suggest a role for IL-15 in MS pathophysiology.