The aim of our study was to estimate the prevalence and severity of allergic rhinitis, rhinoconjunctivitis, and eczema in Costa Rican schoolchildren. We have reported asthma prevalence in the same cohort elsewhere. A questionnaire based survey was conducted on a random sample of children (n 5 6144), aged 6-7 years and 13-14 years from 75 educational centers throughout Costa Rica, using the International Study of Asthma and Allergies in Childhood (ISAAC) protocol. The prevalence in the 6-to 7-year-old group of rhinitis ever was 32.7%, rhinitis in the last 12 months 26.6%, and rhinoconjunctivitis 12.1%. In the 13-to 14-yearold group, the presence of rhinitis ever was 39.2%, rhinitis in the last 12 months 30.9% and rhinoconjunctivitis 14.8, showing significantly higher figures for the older group (p 5 0.001, p 5 0.0001, p 5 0.001). The prevalence of "ever having had a diagnosis of rhinitis" (by a physician) was 5.9% and 4.8% in the two age groups (NS). Children were more affected during the rainy season, May to October. The presence of rash ever was found in 27.2% and 26.3% (NS) in the two age groups. Rash in the past year was 18.4% (6-7 years) and 16.5% (13-14 years) ( p 5 0.04, OR 1.14), while 8.2% and 4.5%, respectively, had had a doctor's diagnosis of eczema ( p , 0.001, OR 1.87). At the age of 6-7 years, boys had rhinitis more often during the previous year, rhinoconjunctivitis and a doctor's diagnosis of rhinitis more often than did girls ( p 5 0.03, p , 0.001, p 5 0.02, p 5 0.001). At the age of 13-14 years the situation reversed ( p 5 0.01, p 5 0.003, p , 0.001, p 5 NS). The symptoms of rhinitis interfered more often with daily activities in the younger group than in the older group. Rhinoconjunctivitis was reported in 24.8% of the younger and in 28.4% of the 13-14 year olds (p 5 NS). Rash in the last 12 months was found in 24.4% of the 6-7 year olds and in 26% of the older group (p 5 NS). The prevalence of asthma, rhinitis, and eczema is high in Costa Rican school children. High humidity in some areas, the high concentrations of airborne allergens, pollution, and high concentrations of mites and cockroaches, due to westernization of bed material, may be responsible. (Pediatr Asthma Allergy Immunol 2004; 17[1]:71-80.)