Regional pollen allergen exposure differences may induce variable sensitization profiles that could affect allergen immunotherapy efficacy and safety. To describe sensitization profiles against timothy grass allergen components (Phl p) in North American subjects screened for a timothy grass sublingual immunotherapy (SLIT) tablet trial and evaluate grass SLIT tablet efficacy and safety based on pretreatment Phl p IgE levels. Serum-specific IgE was measured post hoc by ImmunoCAP ISAC from subjects screened (N = 1,905) for study P08067 (NCT01385371) conducted in Canada and 5 US regions. Subjects exhibited positivity for timothy grass by skin prick testing and serum IgE. Average total combined symptom plus medication score during the entire pollen season and treatment-related adverse events (TRAEs) were determined in randomized subjects (n = 1,140) by pretreatment Phl p IgE levels (group 1, ≤33rd percentile; group 2, >33rd-67th percentile; group 3, >67th percentile; or undetectable). Most screened subjects were sensitized to Phl p 1 (73%) and Phl p 5 (51%). The highest mean IgE levels and largest proportions of subjects with positive reactions for Phl p 1 and Phl p 5 were found in Canada and the western United States. Improvements in total combined symptom plus medication score vs placebo by Phl p 5 IgE groups 1 to 3 were 7.7%, 23.9%, and 35.4%, respectively, and 18.7% for subjects with undetectable Phl p 5 IgE. TRAE incidences with the grass SLIT tablet by Phl p 5 IgE groups 1 to 3 were 56.1%, 66.4%, and 74.5%, respectively, and 49.7% in subjects with undetectable Phl p 5 IgE. Similar, but less pronounced, trends for efficacy and TRAEs were observed for Phl p 1 and Phl p 6 IgE. Sensitization profiles varied by region. Trends toward higher efficacy and increased TRAE incidence in subjects with higher pretreatment Phl p IgE levels were observed. www.clinicaltrials.gov, identifier NCT01385371.