• Average morning nasal obstruction score calculated from diary assessments during treatment. [ Time Frame: 4 week treatment period ] [ Designated as safety issue: No ]
  

• average morning and average evening single symptom score calculated from diary assessments during treatment period (with exception of the primary endpoint) [ Time Frame: 4 week treatment period ] [ Designated as safety issue: No ]
  
• average of the daily mean single symptom score (mean of morning and evening assessment) calculated with respect to diary assessments during treatment period [ Time Frame: 4 week treatment period ] [ Designated as safety issue: No ]
  
• average morning, evening and total T5SS calculated with respect to the treatment period [ Time Frame: 4 week treatment period ] [ Designated as safety issue: No ]
  
• average use of rescue medication (number of applications) during the treatment period [ Time Frame: 4 week treatment period ] [ Designated as safety issue: No ]
  
• differences between the preceding parameters and the respective averages calculated in regard to the follow-up period [ Time Frame: 4 week treatment and 2 week follow-up period ] [ Designated as safety issue: No ]
  
• results and changes in results of questionnaire 1 [ Time Frame: assessed on visits during the 7 week study period ] [ Designated as safety issue: No ]
  
• results of questionnaires 2, 3 and 4 [ Time Frame: assessed on visits during the 4 week treatment and the 2 week follow-up period ] [ Designated as safety issue: No ]
  

The study is performed according to a monocentre, double-blind, placebo controlled, two arm parallel group design. It is divided in 3 periods: A 7 ± 2 day screening phase when patients are not to treat their perennial allergic rhinitis (PAR) is followed by a 29 ± 4 day treatment period when patients administer either levocetirizine 5 mg OD or placebo and a 14 ± 2 week post treatment observation period when again medication against PAR is not to be taken. However in all study periods patients are provided cromoglicine nasal spray and eye drops which in case of severe complaints they may use as rescue medication.

Throughout the study patients keep a diary documenting the severity of their nasal and ocular symptoms of allergy. Every morning and every evening they use a 4-point scale (0 = no symptom, 1 = mild, 2 = moderate, 3 = severe) to rate nasal obstruction, rhinorrhea, nasal itching, sneezing and ocular complaints they experienced during the preceding 12 hours. Furthermore they report adverse events and intake of drugs inclusive of rescue and (during treatment period) study medication.

Patients attend 5 visits to the study site. On visit 1 medical history and concomitant medication are assessed and patients undergo a physical examination, a pregnancy test in case of women of child bearing potential, and, if necessary, a skin prick test. Blood is taken to estimate creatinine clearance. Inclusion and exclusion criteria are checked and, if they do not contradict study continuation, patients enter the screening period which ends in the morning of the day visit 2. If on this visit morning diary reports show sufficiently high nasal obstruction scores and study participation still complies with the inclusion and exclusion criteria, patients are randomized and start the treatment period taking the first dose at the study site. On visit 2 as well as on the following visits adverse events and use of medication are surveyed and compliance is checked. Visit 3, 4 and 5 are scheduled after one week of treatment, between treatment and post treatment observation period and at the end of the study respectively. On visit 5 there is a final physical examination and again females of childbearing potential undergo a pregnancy test. To assess how relevant for patients nasal obstruction and the effect of the study medication on nasal obstruction are questionnaires are completed on each visit.