Anderson HR [Department of Clinical Epidemiology and Social Medicine, St. George's Hospital Medical School, Cranmer Terrace, London SW17 ORE], Bailey PA and Bland JM. The effect of birth month on asthma, eczema hayfever, respiratory symptoms, lung function and hospital admissions for asthma. International Journal of Epidemiology 1981, 10: 45–51. The relationship between month of birth and asthma, respiratory symptoms and peak expiratory flow rate has been investigated in population surveys of children and in hospital admission data. Several relationships with month of birth were found, but these were not consistent. We conclude that a real relationship between month of birth and various respiratory problems is likely. The mechanism remains obscure.