Background There are presently no available therapeutic options for patients with peanut allergy. Objective We sought to investigate the safety, efficacy, and immunologic effects of peanut sublingual immunotherapy (SLIT). Methods After a baseline oral food challenge (OFC) of up to 2 g of peanut powder (approximately 50% protein; median successfully consumed dose SCD, 46 mg), 40 subjects, aged 12 to 37 years (median, 15 years), were randomized 1:1 across 5 sites to daily peanut or placebo SLIT. A 5-g OFC was performed after 44 weeks, followed by unblinding; placebo-treated subjects then crossed over to higher dose peanut SLIT, followed by a subsequent crossover Week 44 5-g OFC. Week 44 OFCs from both groups were compared with baseline OFCs; subjects successfully consuming 5 g or at least 10-fold more peanut powder than the baseline OFC threshold were considered responders. Results After 44 weeks of SLIT, 14 (70%) of 20 subjects receiving peanut SLIT were responders compared with 3 (15%) of 20 subjects receiving placebo ( P  < .001). In peanut SLIT responders, median SCD increased from 3.5 to 496 mg. After 68 weeks of SLIT, median SCD significantly increased to 996 mg (compared with Week 44, P  = .05). The median SCD at the Week 44 Crossover OFC was significantly higher than baseline (603 vs 71 mg, P  = .02). Seven (44%) of 16 crossover subjects were responders; median SCD increased from 21 to 496 mg among responders. Of 10,855 peanut doses through the Week 44 OFCs, 63.1% were symptom free; excluding oral-pharyngeal symptoms, 95.2% were symptom free. Conclusions Peanut SLIT safely induced a modest level of desensitization in a majority of subjects compared with placebo. Longer duration of therapy showed statistically significant increases in the SCD.