Dampness and moisture problems in a building may cause growth of moulds, leading to sensitization and symptoms in the inhabitants. The mechanism by which sensitization to moulds takes place has remained obscure; in particular, the role of atopy is not clear. In 1996, 622 pupils (7-13 years of age) attending a school with a moisture problem (index school; 414 pupils) and a control school (208 pupils) were screened using a questionnaire. Two-hundred and twelve children had doctor-diagnosed asthma, parental-reported wheezing or prolonged cough, and they participated in a clinical study, which included skin prick tests (SPT) to 12 moulds. An identical, follow-up study was performed 3 years later in 1999. In the follow-up study, 144 of the original 212 students participated. They were now attending four different schools: the index primary school had been renovated and the control school remained unchanged, but the two secondary schools had moisture and mould problems. The purpose of the study was to evaluate the occurrence of mould allergy in children of school age and to compare sensitization to moulds in relation to age, exposure, asthma, and atopy. In 1999, SPT responses to moulds were demonstrated in 17 (12%) of the 144 children. Six children had SPT reactions > or = 3 mm and all but one were older than 14 years. During the 3-year follow-up period, mould allergy developed in five children and disappeared in two children. Five of the six children with reactions > or = 3 mm to moulds had positive responses to other allergens, five had clinical atopy but only two had asthma. Likewise, all six children had been exposed to moisture and dampness in the school buildings. In conclusion, mould allergy diagnosed by SPTs was rare in students. Most reactions to moulds were in students older than 14 years with multiple SPT reactions to common allergens, and there was no significant association with asthma.