The background and aetiology of chronic cough were investigated by comparing 60 children under 6 years with simple cough, 60 children with asthma, and 60 controls. Both cough and asthma were more common in boys and associated with a history of eczema, chest deformity, and skin reactivity to inhaled allergens, but these findings were more prevalent in asthma than cough. House dust mite sensitivity was found in 34 (57%) children with cough, 45 (75%) with asthma, and six (10%) controls. Tests of immunological function showed some high concentrations of IgM in groups with both cough and asthma, but high IgE concentrations, eosinophilia, and lymphocytosis were significant only in asthma. IgG1 and IgG2 concentrations were raised in some children with cough or asthma, but the only low subclass concentrations were of IgG3 observed in the group with cough. Children with simple cough represented a heterogeneous population but many showed evidence of atopy. Major defects of immunity were not observed.