Sensitization to house dust mites (HDMs) is frequent in patients with atopic dermatitis. To investigate the efficacy of sublingual immunotherapy (SLIT) with Dermatophagoides pteronyssinus extract in patients with atopic dermatitis sensitized to HDM. In this randomized, double-blind, placebo-controlled trial, we enrolled 91 patients 3 years or older, with SCORing Atopic Dermatitis (SCORAD) score greater than or equal to 15 and positive skin test result and/or IgE to D pteronyssinus. Patients were stratified according to age (<12 and ≥12 years) to receive HDM SLIT or placebo for 18 months. Primary outcome was a greater than or equal to 15-point decrease in SCORAD score. Secondary outcomes were decreases in SCORAD and objective SCORAD, Eczema Area and Severity Index, visual analog scale for symptoms, and pruritus scale scores; Investigator's Global Assessment 0/1; and decrease greater than or equal to 4 points in Dermatology Life Quality Index. Background therapy was maintained. A total of 66 patients completed the study (35 HDM SLIT, 31 placebo). After 18 months, 74.2% and 58% of patients in the HDM SLIT group and the placebo group, respectively, showed greater than or equal to 15-point decrease in SCORAD score (relative risk, 1.28; 95% CI, 0.89-1.83). Significant SCORAD score decreases from baseline of 55.6% and 34.5% in HDM SLIT and placebo groups (mean difference, 20.4; 95% CI, 3.89-37.3), significant objective SCORAD score decreases of 56.8% and 34.9% in HDM SLIT and placebo groups (mean difference, 21.3; 95% CI, 0.66-41.81), and more patients with Investigator's Global Assessment 0/1 in the HDM SLIT group as compared with the placebo group (14 of 35 vs 5 of 31; relative risk, 2.63; 95% CI, 1.09-6.39) were observed at 18 months. Our results suggest that HDM SLIT may be effective in HDM-sensitized patients as an add-on treatment for atopic dermatitis.