Background: The expression of the regulatory cytokines interleukin (IL)-12 and IL-18 in patients with both Th1- and Th2-mediated diseases, type 1 diabetes mellitus (T1DM) and asthma, is unknown. Objective: To investigate the in vivo and in vitro IL-12 and IL-18 secretion patterns in patients with both T1DM and asthma. Methods: Peripheral blood mononuclear cells (PBMC) were collected from 44 patients. Mean age 19.4 ± 4.7 yr (10.5-28 yr), divided into four paired groups: T1DM and asthma, asthma only, T1DM only, and healthy controls. T-cell proliferative response was assessed. IL-12 and IL-18 serum levels and expression by PBMC following in vitro stimulation by lipopolysaccharide (LPS) were determined by enzyme-linked immunosorbent assay (ELISA). Results: Patients with T1DM and asthma had higher serum levels of both IL-12 and IL-18 compared to controls: 146.2 ± 69.2 and 109.7 ± 34.6 pg/mL, p = 0.038 and 436.1 ± 117.9, 320.2 ± 99.1 pg/mL, p = 0.028, respectively. Stimulated IL-12 secretion was significantly lower in these patients compared to those with one disease only: 809 ± 426.4, 2111.6 ± 2214.3, 3188.1 ± 2692.9 pg/mL and after 48 h: 956.3 ± 489.3, 2429.8 ± 2394.6, 3874.5 ± 2820.3 pg/mL, respectively, p < 0.03 for all. The IL-18/IL-12 serum ratio was also significantly higher in patients with both diseases compared to those with asthma only, p = 0.017. Conclusion: Patients with both T1DM and asthma display a different pattern of IL-12 and IL-18 expression compared to patients with one disease only and controls.