BACKGROUND Allergy is one of the most common causes of respiratory symptoms in children and youth. OBJECTIVE Evaluate the presence and the type of allergic sensitization in a paediatric population with respiratory symptoms. METHODS We studied 564 consecutive children, 5 months to 17 years of age, with a male to female ratio (M/F) = 1.4, referred to our outpatient clinic in a 12-month period retrospectively. Patients were arbitrarily divided into four groups (grs) according to their age: gr1 = 5 months to 4 years old (181 patients), gr2 = 4 to 7 years (201 patients), gr3 = 7 to 10 years (96 patients), and gr4 = 10 to 17 years (86 patients). Sensitization to house dust mites, pollens, animal dander, and molds was determined by skin prick testing. RESULTS Sensitization to at least one class of allergen occurred in 304 of the 564 patients (53.9%, M/F ratio = 2.0); the percentage of allergic patients increased with age as follows: 29.8% (54 patients) of the patients in gr1, 55.2% (111 patients) in gr2, 68.8% (66 patients) in gr3 and 84.9% (73 patients) in gr4 (chi(2) = 84.1, P < .01). In the entire allergic population and in gr1 to gr3, the most common positive allergic reaction was to house dust mites (P < .01, chi(2) test each comparison). In contrast, gr4 patients showed a nearly equal percentage of sensitization to pollens and to house dust mites (79.5% and 78.1% respectively) (chi(2) = 0.0, P = >.1). Sensitization to only one class of allergen occurred in 51.3% of the allergic patients and the percentage of these monosensitized patients tended to decrease from gr1 to gr4 (chi(2) = 15.2, P < .1). In the monosensitized group, sensitization to house dust mites was the most frequent in gr1 to gr3 (age <10 years) as in the whole sample. In gr4, the frequency of sensitization to house dust mites was similar to that of sensitization to pollens. On the contrary, within the patient group sensitized to two or more allergens (polysensitized patients), sensitization to house dust mites was as frequent as sensitization to pollens already in gr2 as compared with monosensitized patients. CONCLUSIONS In children with respiratory symptoms, the percentage of allergic individuals was high and increased with the age of the patients. This phenomenon was associated with an age-related enhancement in the ratio of polysensitized to monosensitized patients and with an age-related increase in the frequency of sensitization to seasonal allergens (ie, pollens).