Background: IL-35 has been found to be involved in many inflammatory diseases in humans but its role in asthma and chronic obstructive pulmonary disease (COPD) is not clear. The plasma level of IL-35 in patients with asthma and COPD needs to be measured. Objective: The aim of this study was to examine the plasma concentrations of IL-35 in newly diagnosed asthmatic and COPD patients and control subjects and investigate correlations of lung function, age, sex, smoking history with the levels of IL-35 in plasma in these diseases. Methods: Blood samples were collected from patients with newly diagnosed asthma (44, 12 males, aged 33.75 +/- 8.94), newly diagnosed COPD (36, 36 males, aged 68.03 +/- 8.94), and healthy control groups (23, 9 males, aged 30.06 +/- 7.50). We determined the IL-35 levels in plasma by enzyme-linked immunosorbent assay. Result: The median and the range of values for IL-35 were 118.55 pg/ml (range 74.43 similar to 1767.22 pg/ml) in patients with asthma, 136.09 pg/ml (range 62.54 similar to 697.49 pg/ml) in patients with COPD and 275.86 pg/ml (range 26.11 similar to 4766.20 pg/ml) in control subjects. The levels of IL-35 in plasma showed a positive correlation with FEV1% and FVC% in asthmatic patients whose plasma IL-35 values were over 150 pg/ml. A positive correlation was also found between plasma IL-35 and FVC% in COPD patients whose plasma IL-35 values were over 150 pg/ml. Conclusions: These findings suggest that IL-35 may very probably be involved in the Th2 and Th17 mediated inflammation process of asthma and COPD. Its role in the mechanisms of COPD and asthma in human beings, as well as its therapeutic value in these diseases, need further investigation.