The purpose of this paper was to evaluate the levels of IL-9 in patients with SLE and RA compared with controls and the association of IL-9 levels with clinical and laboratory parameters. IL-9 levels were assessed in 117 SLE patients, 67 RA patients, and 24 healthy controls by ELISA. Clinical and laboratory parameters were recorded. The IL-9 serum levels were significantly higher in RA patients (4,77 ± 3,618 pg/mL) and in SLE patients (12,26 ± 25,235 pg/mL) than in healthy individuals (1,22 ± 0,706 pg/mL) (p < 0,001). In SLE patients, there were no statistically significant associations or correlations between the levels of IL-9 and SLEDAI or other clinical and laboratorial parameters, with the exception of disease time, which showed a statistically significant negative correlation with IL-9 levels (r = -0,1948; p = 0,0378). In RA patients, no association or statistically significant correlation was observed with disease duration, DAS28, HAQ, rheumatoid factor positivity, or erosions on radiography. These data demonstrated increased serum levels of IL-9 in SLE and RA patients, but further studies are needed to clarify the precise role of this cytokine and its potential use as therapeutic target.