The association of birth month with the development of atopic and non-atopic rhinitis was investigated in 327 adults in south-west London. Birth month was a significant risk factor for both atopic and non-atopic rhinitis in subjects whose age at onset was under 20 yr. but not in those who were older at onset. The odds of being born in May June were 1·68(95%CI: 1·03–2·76) for atopic rhinitic subjects and 1·90 (95% CI: 1·11–3·25) for non-atopic rhinitic subjects. The birth month effect was not related to the criteria used to determine subjects “atopic status or to subjects” clinical characteristics, apart from their age at onset. The findings suggest that atopic and non-atopic rhinitis have a common aetiology which does not depend on infant sensitization to seasonal allergens.