As interleukin-15 (IL-15) has been implicated in the pathophysiology of rheumatoid arthritis, we analysed the serum IL-15 (sIL-15) levels in healthy subjects and patients with early arthritis to establish a cut-off point that might serve to define elevated sIL-15. This is an initial step to determine whether sIL-15 has the potential for use as a biomarker for patients with early arthritis. The IL-15 concentration was measured in serum obtained from 161 healthy controls and from 174 patients with early arthritis, and the relationship between the expression of the two IL-15 mRNA variants and the sIL-15 levels was also assessed. In healthy controls, the median sIL-15 value was 0.83 [interquartile range (IQR) 0-8.68] pg/mL; there was no significant difference in the sIL-15 values according to gender [median level in males was 1.99 (IQR: 0-8.68) pg/mL and in females 0.50 (0-8.25) pg/mL: p = 0.821]. Moreover, sIL-15 levels did not correlate with age (r = 0.033, p = 0.685), and they did not display a clear circadian rhythm in healthy donors, with the median values for IL-15 close to zero at each time tested. In the light of these findings, we considered that sIL-15 was elevated if its concentration was above 20 pg/mL, since this cut-off point corresponded to the 90th percentile for this healthy population. We found that 30% of the patients with early arthritis had sIL-15 values > 20 pg/mL. The levels of sIL-15 did not correlate with disease duration in early arthritis patients, nor did they fluctuate with changes in disease activity over the follow-up period. In addition, the high level of sIL15 in patients was not associated with alterations in the alternative splicing of the IL-15 mRNA, favouring the variant that produces the protein with a long signal peptide for secretion. Serum IL-15 levels were increased in a subpopulation of patients with early arthritis, indicating that this measure may serve as a biomarker for this condition. Further studies will be necessary to determine whether the clinical evolution or response to treatment of patients with high sIL-15 levels differs.