The evidence of the effect of the age at introduction of new foods during infancy on the development of asthma and allergic rhinitis is inconsistent and scarce. We set out to study these associations. A prospective birth cohort of infants with increased HLA-DQB1-conferred risk for type 1 diabetes was recruited in 1996–2000. The families completed at home a record on the age at introduction of new foods. Persistent asthma and allergic rhinitis were assessed at the age of 5 years with an International Study of Asthma and Allergies in Childhood-type questionnaire. The Cox proportional hazards regression analyses were adjusted for parental asthma and allergic diseases, and several perinatal and sociodemographical factors. Out of the 1293 children, 77 (6·0 %) developed persistent asthma; and out of the 1288 children, 185 (14·4 %) developed allergic rhinitis by the age of 5 years. Early age at introduction of oats was associated with a reduced risk of persistent asthma (hazard ratio (HR; 95 % CI) for the first and mid-tertiles compared with the latest tertile was 0·36 (0·15, 0·85) and 0·37 (0·22, 0·62), respectively, P < 0·001). Early age at introduction of fish was dose dependently associated with a decreased risk of allergic rhinitis (HR (95 % CI) for the first and mid-tertiles compared with the latest tertile was 0·34 (0·22, 0·54) and 0·45 (0·28, 0·70), respectively, P < 0·001). The present finding that age at introduction of oats is inversely and independently associated with development of persistent asthma is novel. We confirmed the earlier observation that the age at introduction of fish is inversely related to the risk of allergic rhinitis. Clinical implications remain to be determined.