The purpose of this trial is to investigate the tolerability of Osiris Phleum pratense used with 2 simplified up-dosing schedules compared to the up-dosing schedule used in current practice.

Inclusion Criteria:

• Written informed consent obtained before entering the trial
• Male or female >/= 18 years at visit 1
• A clinically relevant history of grass pollen induced allergic rhinoconjunctivitis (moderate to severe) and having received symptomatic treatment during grass pollen season 2010 and 2011
• Positive skin prick test response (wheal diameter >/= 3mm) to Phleum pratense
• Positive specific IgE against Phleum pratense (>/= 0,70KUL / class 2)
• Female subjects of childbearing potential must have a negative pregnancy test and be willing to practice appropriate contraceptive methods until Visit 4
• Subjects willing and able to comply with trial protocol regimen

Exclusion Criteria:

• Subjects included in another protocol (treatment intervention and/or investigational medicine product) or having participated in another clinical trial within 30 days prior to visit 1
• A clinically relevant history of symptomatic seasonal allergic rhinoconjunctivitis caused by an allergen (e.g. hazel, alder, birch, ash) to which the subject will be exposed during the 30-day treatment period.
• A clinically relevant medical history of symptomatic perennial allergy to allergen(s) to which the subject is regularly exposed (e.g. cat, house dust mites).
• Known sensitization (history of positive SPT) to food allergens with oral allergy syndrome
• Uncontrolled asthma (in accordance with GINA guidelines) within the last 12 months
• FEV < 60% of predicted within the last 12 months
• Severe asthma exacerbation(s) within the last 12 months
• A clinically relevant chronic disease (>/= 3 months) (e.g fibrosis, malignancy, type 1 diabetes mellitus, malabsorption or malnutrition, renal or hepatic insufficiency)
• Malignancy or systemic disease affecting the immune system (e.g. autoimmune disease, immune complex disease or immune deficiency disease)
• Inflammatory conditions in the oral cavity with severe symptoms such as oral lichen planus with ulcerations or severe oral mycosis or dental extraction at randomisation
• Medical history of recurrent urticaria or atopic dermatitis during the last 2 years
• Currently receiving treatment preventing the initiation of SIT (e.g. tricyclic antidepressants, mono amine oxidase inhibitors (MAOIs) and catechol-O-methyl transferase inhibitors (COMT inhibitors))
• History of allergy, hypersensitivity, or intolerance to the excipients of the investigational medicinal product
• Being immediate family of the investigator or trial staff, defined as the investigator's / staff's spouse, parent, grandparent, child or grandchild
• History of drug induced (incl. immunotherapy) facial angioedema (including experience of Quincke oedema) or a family (parents or siblings) history of hereditary angioedema
• Anticipated use of any prohibited medication within the specified time windows as defined in the protocol
• Previous treatment by immunotherapy with grass pollen for more than one month within the last 5 years
• Any clinically significant condition or situation, other than the condition being studied, that in the opinion of the investigator would interfere with the trial evaluations or optimal participation
• History of anaphylaxis with cardio respiratory symptoms (e.g. food allergy, drugs or an idiopathic reaction)