Background: Climatic changes causing early pollen flight and new allergens prolonging the pollen season render up-dosing of allergen-specific subcutaneous immunotherapy (SCIT) outside the pollen season considerably more difficult. In addition, for patients with multiple pollen allergies, patients coming near the beginning of pollen season, and patients who wish to up-dose faster, an accelerated induction regimen would be helpful. Methods: In an open, randomized, parallel group, multicenter safety trial, an accelerated up-dosing regimen (0.1-0.3-0.5 ml in weekly intervals) was compared to conventional up-dosing (0.05-0.1-0.2-0.3-0.4-0.5 ml in weekly intervals) with an allergoid grass pollen SCIT preparation. After up-dosing, the maintenance dose was given in monthly intervals. Results: A total of 146 adult patients with rhinitis or rhinoconjunctivitis with or without mild asthma (FEV1 >70%) due to grass pollen were randomized to either the conventional registered up-dosing or an accelerated regimen. In both groups (accelerated regimen, n Euro Surveillance (Bulletin Europeen Sur Les Maladies Transmissibles; European Communicable Disease Bulletin) 69; conventional regimen, n Euro Surveillance (Bulletin Europeen Sur Les Maladies Transmissibles; European Communicable Disease Bulletin) 75), a high proportion of patients (92.75 and 92.0%, respectively) successfully reached the maintenance dose without safety concerns. Furthermore, significant increases in specific IgG and IgG4 after 4 months of treatment were observed in both groups. Conclusion: The accelerated SCIT regimen was found to be as safe as the conventional regimen and might be used to up-dose patients within 2 weeks. Moreover, the immunological effects of both up-dosing regimens were comparable.