BACKGROUND Children attending a primary school showed symptoms such as headache, cough, rhinitis and epistaxis. Assessment of specific IgE to formaldehyde gave positive results in some children. OBJECTIVE Was IgE-mediated sensitization as well as symptoms in children associated with formaldehyde exposure at school? METHODS Sixty-two 8-year-old children attending three forms at a primary school were investigated. Indoor formaldehyde concentrations were measured in classrooms of both schools (one frame construction with particleboard used extensively as panelling vs a brick building) which were consecutively attended. Assessment of specific IgE to formaldehyde was done in all children. Children were transferred to a brick building and 3 months later specific IgE to formaldehyde in pupils showing initially elevated radioallergosorbent test (RAST) values reassessed. In all children symptoms were evaluated by questionnaire before and 3 months after changing school. RESULTS In the school panelled with particleboard the World Health Organization (WHO) threshold for formaldehyde of 0.050 ppm was crossed in two classrooms (0.075 ppm and 0.069 ppm) whereas in one classroom 0.043 ppm was found. RAST classes of > or = 2 were found in three children, two of them attending the classroom with 0.075 ppm formaldehyde. Elevated RAST classes of > or = 1.3 were found in another 21 pupils. Thirty-eight pupils as well as 19 control children showed RAST classes in the normal range of < or = 1.2. Headache, nose bleeding, rhinitis, fatigue, cough, dry nasal mucosa and burning eyes were found in the affected children. There was a good correlation between symptoms and the formaldehyde concentrations in the classrooms. However, elevated IgE levels to formaldehyde did not correlate with symptoms. Formaldehyde concentrations in the classrooms of the brick built school were 0.029 ppm, 0.023 ppm and 0.026 ppm. After transferral specific IgE to formaldehyde decreased significantly from 1.7 +/- 0.5 to 1.2 +/- 0.2 (P < 0.002) as did the incidence of symptoms. CONCLUSIONS Gaseous formaldehyde, besides its irritant action, leads to IgE-mediated sensitization. As children are more sensitive to toxic substances than adults, threshold levels for indoor formaldehyde should be reduced for children.