We have evaluated the effectiveness of cetirizine in 40 patients (25 males and 15 females, mean age 32.8 yrs) suffering from perennial allergic rhinitis due to Dermatophagoides pteronyssinus. Patients were treated for 30 days with oral cetirizine (10 mg once a day) and had to report the severity of the symptoms considered (sneezing, runny nose, itching and congestion) on daily card. During the study no other medication was allowed. After 4 weeks symptoms were statistically improved and this beneficial effect increased at the end of the treatment. No side effects were reported. Oral cetirizine is therefore a useful treatment in perennial allergic rhinitis due to Dermatophagoides pteronyssinus.