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.