Of 1218 children born on the Isle of Wight in 1989/90, and followed for atopy at age 4 years, 981 were skin-prick tested witha battery of allergens. Of these 61 (6%) reacted positively to Alternaria alternata and Cladosporium herbarum (47 to Alternaria, 21 to Cladosporium and seven to both). Twenty-four (39%) were asymptomatic (latent atopy) of which 12 had a single positive reaction either to Alternaria or Cladosporium. Asthma was the most common disease in children sensitized to moulds, Alternaria sensitization correlated positively with clinical diagnosis of asthma (P < 0.01), eczema (P <0.001) and rhinitis (P <0.05), Likewise, Cladosporium sensitivity correlated with a diagnoses of asthma, eczema and rhinitis (all P <0.05). Age of the house correlated with reported damp and lack of central heating (both P <0.001), but not with sensitization to moulds. An association between the presence of damp or age of the house and mould allergy was confounded by 21 children moving house in the first 4 years. Exposure to pets, passive tobacco smoking and season of birth had no bearing on mould sensitivity. At 4 years of age Alternaria and Cladosporium were the third most common causes of sensitization, i.e. after house dust mite and grass pollen.